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THE ORIGIN OF VERTEBRATES

WALTER HOLBROOK GASKELL

THE

ORIGIN OF VERTEBRATES

THE

ORIGIN OF VERTEBRATES

WALTER HOLBROOK GASKELL

M.A., M.D. (CANTAB.), LL.D. (EDIN. AND McGILL UNIV.) ; F.R.S. ; FELLOW OF TRINITY
HALL AND UNIVERSITY LECTURER IN PHYSIOLOGY, CAMBRIDGE ; HONORARY FELLOW
OF THE ROYAL MEDICAL AND CHIRURGICAL SOCIETY; CORRESPONDING MEMBER
OF THE IMPERIAL MILITARY ACADEMY OF MEDICINE, ST. PETERSBURG, ETC.

LONGMANS, GREEN, AND CO

39 PATERNOSTER ROW, LONDON

NEW YORK, BOMBAY, AND CALCUTTA

1908

CONTENTS

PAGE

Introduction 1

CHAPTER I
The Evidence of the Central Nervous System

Theories of the origin of vertebrates — Importance of the central nervous system
— Evolution of tissues — Evidence of Paleontology — Reasons for choosing
Ammoccetes rather than Amphiosus for the investigation of this problem —
Importance of larval forms — Comparison of the vertebrate and arthropod
central nervous systems — Antagonism between cephalization and alimenta-
tion— Life-history of lamprey, not a degenerate animal — Brain of Ammo-
ccetes compared with brain of arthropod — Summary . . ■ . . 8

CHAPTER II

The Evidence of the Organs of Vision

Different kinds of eye — Simple and compound retinas — Upright and inverted
retinas — Median eyes — Median or pineal eyes of Ammoccetes and their
optic ganglia — Comparison with other median eyes — Lateral eyes of verte-
brates compared with lateral eyes of crustaceans — Peculiarities of the
lateral eye of the lamprey — Meaning of the optic diverticula — Evolution
of vertebrate eyes — Summary 68

CHAPTER III

The Evidence of the Skeleton

The bony and cartilaginous skeleton considered, not the notochord — Nature of
the earliest cartilaginous skeleton — The mesosomatic skeleton of Ammo-
ccetes ; its topographical arrangement, its structure, its origin in muco-
cartilage — The prosomatic skeleton of Ammoccetes ; the trabeculse and
parachordals, their structure, their origin in white fibrous tissue — The
mesosomatic skeleton of Limulus compared with that of Ammoccetes ;
similarity of position, of structure, of origin in muco-cartilage — The
prosomatic skeleton of Limulus ; the entosternite, or plastron, compared
with the trabeculse of Arnmocoetes ; similarity of position, of structure, of
origin in fibrous tissue — Summary 119

31233

vi CONTENTS

CHAPTER IV
The Evidence of the Respiratory Apparatus

r.\r, i.

Branchiae considered as internal branchial appendages — Innervation of branchial
segments — Cranial region older than spinal — Three-root system of cranial
nerves : dorsal, lateral, ventral — Explanation of van Wijhe's segments —
Lateral mixed root is appendage-nerve of invertebrate — The branchial
chamber of Amniocoetes — The branchial unit, not a pouch but an appendage
— The origin of the branchial musculature — The branchial circulation — The
branchial heart of the vertebrate — Not homologous with the systemic heart
of the arthropod — Its formation from two longitudinal venous sinuses —
Summary 148

CHAPTER V
The Evidence of the Thyroid Gland

The value of the appendage-unit in non-branchial segments — The double nature
of the hyoid segment — Its branchial part — Its thyroid part — The double
nature of the opercular appendage — Its branchial part — Its genital part-
Unique character of the thyroid gland of Ammoccetes — Its structure-
Its openings — The nature of the thyroid segment — The uterus of the
scorpion — Its glands — Comparison with the thyroid gland of Ammoccetes—
Cephalic generative glands of Limulus — Interpretation of glandular tissue
filling up the brain-case of Ammoccetes — Function of thyroid gland —
Relation of thyroid gland to sexual functions — Summary .... 185

CHAPTER VI

The Evidence of the Olfactory Apparatus

Fishes divided into Amphirhime and Monorhins — Nasal tube of the lamprey
—Its termination at the infundibulum — The olfactory organs of the scorpion
group — The camerostome — Its formation as a tube — Its derivation from a
pair of antennae — Its termination at the true mouth — Comparison with the
olfactory tube of Ammoccetes — Origin of the nasal tube of Ammoccetes from
the tube of the hypophysis — Direct comparison of the hypophysial tube with
the olfactory tube of the scorpion group — Summary 218

CHAPTER VII

The Prosomatic Segments of Limulus and its Allies

Comparison of the trigeminal with the prosomatic region — The prosomatic
appendages of the Gigantostraca — Their number and nature — Endognaths
and ectognath — The metastoma — The coxal glands — Prosomatic region of
Eurypterus compared with that of Ammoccetes — Prosomatic segmentation
shown by marks on carapace — Evidence of ccelomic cavities in Limulus —
Summary 233

CONTENTS vii

CHAPTER VIII

The Segments belonging to the Trigeminal Nerve-Group

PAGE

The prosoruatic segments of the vertebrate — Number of segments belonging to
the trigeminal nerve-group — History of cranial segments — Eye-muscles and
their nerves — Comparison with the dorso-ventral somatic muscles of the
scorpion — Explanation of the oculomotor nerve and its group of muscles —
Explanation of the trochlear nerve and its dorsal crossing — Explanation
of the abducens nerve — Number of segments supplied by the trigeminal
nerves — Evidence of their motor nuclei — Evidence of their sensory ganglia
— Summary 257

CHAPTER IX

The Prosomatic Segments op Ammoccetes

The prosomatic region in Ammoccetes — The suctorial apparatus of the adult
Petromyzon — Its origin in Ammoccetes — Its derivation from appendages —
The segment of the lower lip or the metastomal segment — The tentacular
• segments — The tubular muscles — Their segmental arrangement — Their
peculiar innervation — Their correspondence with the system of veno-peri-
cardial muscles in Limulus — The old mouth or palseostoma — The pituitary
gland — Its comparison with the coxal gland of Limulus — Summary . . 286

CHAPTER X

The Relationship of Ammocostes to the most Ancient Fishes

— the Ostracodermata

The nose of the Osteostraci — Comparison of head-shield of Ammoccetes and of
Cephalaspis — Ammoccetes only living representative of these ancient fishes
— Formation of cranium — Closure of old mouth — Rohon's primordial
cranium — Primordial cranium of Phrynus and Galeodes — Summary . . 326

CHAPTER XI

The Evidence of the Auditory Apparatus and the Organs of

the Lateral Line

Lateral line organs — Function of this group of organs — Poriferous sense-organs
on the appendages in Limulus — Branchial sense-organs — Prosomatic sense-
organs — Flabellum — Its structure and position — Sense-organs of mandibles
— Auditory organs of insects and arachnids — Poriferous chordotonal organs —
Balancers of Diptera — Resemblance to organs of flabellum — Racquet-organs
of Galeodes — Pectens of scorpions — Large size- of nerve to all these special
sense-organs — Origin of parachordals and auditory capsule— Reason why
VHth nerve passes in and out of capsule — Evidence of Ammoccetes —
Intrusion of glandular mass round brain into auditory capsule — Intrusion
of generative and hepatic mass round brain into base of flabellum —
Summary 355

viii CONTENTS

CHAPTER XII
The Region op the Spinal Cord

PAGE

Difference between cranial and spinal regions — Absence of lateral root — Meristic
variation — Segmentation of coelorn — Segmental excretory organs — Develop-
ment of nepbric organs ; pronepbric, mesonepbric, metanepbric — Excretory
organs of Ampbioxus— Solenocytes — Excretory organs of Brancbipus and
Peripatus, appendicular and somatic — Comparison of coelom of Peripatus
and of vertebrate — Pronepbric organs compared to coxal glands — Origin of
vertebrate body-cavity (metaccele) — -Segmental duct — Summary of formation
of excretory organs — Origin of somatic trunk-musculature — Atrial cavity
of Ampbioxus — Pleural folds — Ventral growtb of pleural folds and somatic
musculature — Pleural folds of Cephalaspidse and of Trilobita — Meaning of
tbe ductless glands — Alteration in structure of excretory organs which bave
lost tbeir duct in vertebrates and in invertebrates — Formation of lympbatic
glands — Segmental coxal glands of arthropods and of vertebrates — Origin of
adrenals, pituitary body, tbymus, tonsils, thyroid, and other ductless glands
— Summary 385

CHAPTER XIII
The Notochord and Alimentary Canal

Relationship between notocbord and gut — Position of unsegmented tube of
notocbord — Origin of notocbord from a median groove — Its function as an
accessory digestive tube — Formation of notocbordal tissue in invertebrates
from closed portions of tbe digestive tube — Digestive power of tbe skin of
Ammoccetes — Formation of new gut in Ammoccetes at transformation —
Innervation of the vertebrate gut — The three outflows of efferent nerves
belonging to vtbe organic system — The original close contiguity of the
respiratory chamber to the cloaca — The elongation of the gut — Conclusion 433

CHAPTER XIV

The Principles of Embryology

The law of recapitulation — Vindication of this law by tbe theory advanced in
this book — The germ-layer theory — Its present position — A physiological
not a morphological conception — New fundamental law required — Com-
position of adult body — Neuro-epitbelial syncytium and free-living cells —
Meaning of the blastula — Derivation of the Metazoa from the Protozoa —
Importance of the central nervous system for Ontogeny as well as for
Phylogeny — Derivation of free-living cells from germ-cells — Meaning of
coelom — Formation of neural canal — Gastrula of Ampbioxus and of Lucifer
— Summary 455

CONTENTS ix

CHAPTER XV
Final Remarks

PAGE

Problems requiring investigation — ■

Giant nerve-cells and giant nerve-fibres ; tbeir comparison in fisbes and
artbropods ; blood- and lymph-corpuscles ; nature of the skin ; origin of
system of unstriped muscles ; origin of the sympathetic nervous system ;
biological test of relationship.

Criticisms of Balanoglossus theory — Theory of parallel development — Importance

of the theory advocated in this book for all problems of Evolution . . 488

Bibliography and Index of Authors 501

General Index 517

" GO ON AND PROSPER ; THERE IS NOTHING SO
USEFUL IN SCIENCE AS ONE OF THOSE EARTH-
QUAKE HYPOTHESES, WHICH OBLIGE ONE TO FACE
THE POSSIBILITY THAT THE SOLIDEST-LOOKING
STRUCTURES MAY COLLAPSE."

Letter from Prof. Huxley to
the Author. June 2, 1889.

THE

ORIGIN OF VERTEBRATES

INTRODUCTION

In former days it was possible for a man like Johannes Muller
to be a leader both in physiology and in comparative anatomy.
Nowadays all scientific knowledge has increased so largely that
specialization is inevitable, and every investigator is confined more
and more not only to one department of science, but as a rule to
one small portion of that department. In the case of such cognate
sciences as physiology and comparative anatomy this limiting of the
scope of view is especially deleterious, for zoology without physiology
is dead, and physiology in many of its departments without com-
parative anatomy can advance but little. Then, again, the too
exclusive study of one subject always tends to force the mind into
a special groove— into a line of thought so deeply tinged with the
prevalent teaching of the subject, that any suggestions which arise
contrary to such teaching are apt to be dismissed at once as heretical
and not worthy of further thought ; whereas the same suggestion
arising in the mind of one outside this particular line of thought
may give rise to new and valuable scientific discoveries.

Nothing but good can, in my opinion, result from the incursion
of the non-specialist into the realm of the specialist, provided that
the former is in earnest. Over and over again the chemist has
given valuable help to the physicist, and the physicist to the
chemist, so closely allied are the two subjects ; so also is it with
physiology and anatomy, the two subjects are so interdependent
that a worker in the one may give valuable aid towards the solution
of some large problem which is the special territory of the other.

It has been a matter of surprise to many how it came about that

2 THE ORIGIN OF VERTEBRATES

I, a worker in the physiological laboratory at Cambridge ever since
Foster introduced experimental physiology into English-speaking
nations, should have devoted so much time to the promulgation of
a theory of the origin of vertebrates — a subject remote from phy-
siology, and one of the larger questions appertaining to comparative
anatomy. By what process of thought was I led to take up the
consideration of a subject apparently so remote from all my previous
work, and so foreign to the atmosphere of a physiological laboratory ?

It may perhaps be instructive to my readers to see how one
investigation leads to another, until at last, nolens volens, the worker
finds himself in front of a possible solution to a problem far removed
from his original investigation, which by the very magnitude and
importance of it forces him to devote his whole energy and time to
seeing whether his theory is good.

In the years 1880-1884 I was engaged in the investigation of
the action of the heart, and the nature of the nerves which regulate
that action. In the course of that investigation I was struck by the
ease with which it was possible to distinguish between the fibres of
the vagus and accelerator nerves on their way to the heart, owing to
the medullation of the former and the non-medullation of the latter.
This led me to an investigation of the accelerator fibres, to find out
how far they are non-medullated, and so to the discovery that the
rami commicnicantes connecting together the central nervous system
and the sympathetic are in reality single, not double, as had
hitherto been thought ; for the grey ramus communicans is in
reality a peripheral nerve which supplies the blood-vessels of the
spinal cord and its membranes, and is of the same nature as the
grey accelerators to the heart.

This led to the conclusion that there is no give and take
between two independent nervous systems, the cerebro-spinal and
the sympathetic, as had been taught formerly, but only one nervous
system, the cerebro-spinal, which sends special medullated nerve-
fibres, characterized by their smallness, to the cells of the sympathetic
system, from which fibres pass to the periphery, usually non-
medullated. These fine medullated nerves form the system of
white rami communicantcs, and have since been called by Langley
the preganglionic nerves. Further investigation showed that such
white rami are not universally distributed, but are confined to the
thoracico-lumbar region, where their distribution is easily seen in

INTRODUCTION 3

the ventral roots, for the cells of the sympathetic system arc entirely
efferent in nature, not afferent ; therefore, the fibres entering into them
from the central nervous system leave the spinal cord by ventral, not
dorsal roots.

Following out this clue, I then found that in addition to this
thoracico-lumbar outflow of efferent ganglionated visceral nerves,
there are similar outflows in the cranial and sacral regions, belong-
ing in the former case especially to the vagus system of nerves, and
in the latter to the system of nerves which pass from the sacral
region of the cord to the ganglion-cells of the hypogastric plexus,
and from them supply the bladder, rectum, etc. To this system of
nerves, formerly called the nervi erigcntes, I gave the name pelvic
splanchnics, in order to show their uniformity with the abdominal
splanchnics. These investigations led to the conclusion that the
organic system of nerves, characterized by the possession of efferent
nerve- cells situated peripherally, arises from the central nervous
system by three distinct outflows — cranial, thoracico-lumbar, and
sacral, respectively. To this system Langley has lately given the
name ' autonomic' These three outflows are separated by two gaps
just where the plexuses for the anterior and posterior extremities
come in.

This peculiar arrangement of the white rami communicantes set
me thinking, for the gaps corresponded to an increase of somatic
musculature to form the muscles of the fore and hind limbs, so that
if, as seemed probable, the white rami communicantes arise segmentally
from the spinal cord, then a marked distinction must exist in
structure between the spinal cord in the thoracic region, where the
visceral efferent nerves are large in amount and the body muscu-
lature scanty, and in the cervical or lumbar swellings, where the
somatic musculature abounds, and the white rami communicantes
scarcely exist.

I therefore directed my attention in the next place to the
structure of the central nervous system in the endeavour to asso-
ciate the topographical arrangement of cell-groups in this system
with the outflow of the different kinds of nerve-fibres to the
peripheral organs.

This investigation forcibly impressed upon my mind the
uniformity in the arrangement of the central nervous system as far
as the centres of origin of all the segmental nerves are concerned,

4 THE ORIGIN OF VERTEBRATES

both cranial and spinal, and also the original segmental character of
this part of the nervous system.

I could not, therefore, help being struck by the force of the
comparison between the central nervous systems of Vertebrata and
Appendiculata as put forward again and again by the past gene-
• ration of comparative anatomists, and wondered why it had been
discredited. There in the infundibulum was the old oesophagus,
there in the cranial segmental nerves the infracesophageal ganglia,
there in the cerebral hemispheres and optic and olfactory nerves the
supracesophageal ganglia, there in the spinal cord the ventral chain
of ganglia. But if the infundibulum was the old oesophagus, what
then ? The old oesophagus was continuous with and led into the
cephalic stomach. What about the infundibulum ? It was continuous
with and led into the ventricles of the brain, and the whole thing
became clear. The ventricles of the brain were the old cephalic
stomach, and the canal of the spinal cord the long straight intestine
which led originally to the anus, and still in the vertebrate embryo
opens out into the anus. Not having been educated in a morpho-
logical laboratory and taught that the one organ which is homologous
throughout the animal kingdom is the gut, and that therefore the
Efut of the invertebrate ancestor must continue on as the gut of
the vertebrate, the conception that the central nervous system has
grown round and enclosed the original ancestral gut, and that the
vertebrate has formed a new gut did not seem to me so impossible
as to prevent my taking it as a working hypothesis, and seeing to
what it would lead.

This theory that the so-called central nervous system of the
vertebrate is in reality composed of two separate parts, of which
the one, the segmented part, corresponds to the central nervous
system of the highest invertebrates, while the other, the unseg-
mented tube, was originally the alimentary canal of that same
invertebrate, came into my mind in the year 1887. The following
year, on June 23, 1888, I read a paper on the subject before the
Anatomical Society at Cambridge, which was published in the Journal
of Anatomy and Physiology, vol. 23, and more fully in the Journal of
Physiology, vol. 10. Since that time I have been engaged in testing
the theory in every possible way, and have published the results of
my investigations in a series of papers in different journals, a list of
which I append at the end of this introductory chapter.

INTRODUCTION 5

It is now twenty years since the theory first came into my mind,
and the work of those twenty years has convinced me more and more
of its truth, and yet during the whole time it has heen ignored by
the morphological world as a whole rather than criticized. Whatever
may have been the causes for such absence of criticism, it is clear
that the serial character of its publication is a hindrance to criticism
of the theory as a whole, and I hope, therefore, that the publication
of the whole of the twenty years' work in book-form will induce
those who differ from my conclusions to come forward and show me
where I am wrong, and why my theory is untenable. Any one
who has been thinking over any one problem for so long a time
becomes obsessed with the infallibility of his own views, and is not
capable of criticizing his own work as thoroughly as others would
do. I have been told that it is impossible for one man to consider
so vast a subject with that thoroughness which is necessary, before
any theory can be accepted as the true solution of the problem. I
acknowledge the vastness of the task, and feel keenly enough my
own shortcomings. For all that, I do feel that it can only be of
advantage to scientific progress and a help to the solution of this
great problem, to bring together in one book all the facts which I
have been able to collect, which appeal to me as having an important
bearing on this solution.

In this work I have been helped throughout by Miss R. Alcock.
It is not too much to say that without the assistance she has given
me, many an important link in the chain of evidence would have
been missing. With extraordinary patience she has followed, section
by section, the smallest nerves to their destination, and has largely
helped to free the transformation process in the lamprey from the
mystery which has hitherto enveloped it. She has drawn for me
very many of the illustrations scattered through the pages in this
book, and I feel that her aid has been so valuable and so continuous,
lasting as it does over the whole period of the work, that her name
ought fittingly to be associated with mine, if perchance the theory of
the Origin of Vertebrates, advocated in the pages of this book, gains
acceptance.

I am also indebted to Mr. J. Stanley Gardiner and to Dr. A.
Sheridan Lea for valuable assistance in preparing this book for the
press. I desire to express my grateful thanks to the former for
valuable criticism of the scientific evidence which I have brought

6 THE ORIGIN OF VERTEBRATES

forward in this hook, and to the latter for his great kindness in
undertaking the laborious task of correcting the proofs.

LIST OF PREVIOUS PUBLICATIONS BY THE AUTHOR, CON-
CERNING THE ORIGIN OF VERTEBRATES.

1888. "Spinal and Cranial Nerves." Proceedings of the Anatomical Society,

June, 1888. Journal of Anatomy and Physiology, vol. xxiii.

1889. " On the Relation between the Structure, Function. Distribution, and

Origin of the Cranial Nerves ; together with a Theory of the Origin
of the Nervous System of Vertebrata." Journal of Physiology, vol. x.,
p. 153.

1889. •• On the Origin of the Central Nervous System of Vertebrates."

Brain, vol. xii.. p. 1.

1890. '• On the Origin of Vertebrates from a Crustacean-like Ancestor."

Quarterly Journal of Microscopical Science, vol. xxxi.. p. 379.

1895. "The Origin of Vertebrates." Proceedings of the Cambridge Philo-

sophical Society, vol. ix., p. 19.

1896. Presidential Address to Section I. at the meeting- of the British

Association for the Advancement of Science in Liverpool. Report
of the British Association, 1896, p. 942.
1899. " On the Meaning of the Cranial Nerves." Presidential Address to the
Neurological Society for the year 1899. Brain, vol. xxii., p. 329.

A series of papers on " The Origin of Vertebrates, deduced from the
study of Ammocoetes," in the Journal of Anatomy and Physiology, as
follows : —

1898. Part I. " The Origin of the Brain," vol. xxxii., p. 513.

II. " The Origin of the Vertebrate Cranio-facial Skeleton,"
vol. xxxii., p. 553.
III. " The Origin of the Branchial Segmentation," vol. xxxiii..
p. 154.

1899. .. IV. " The Thyroid, or Opercular Segment : the Meaning of the

Facial Nerve," vol. xxxiii.. p. 638.

1900. .. V. " The Origin of the Pro-otic Segmentation : the Meaning

of the Trigeminal and Eye-muscle Nerves," vol. xxxiv..

p. 465.
1900. .. VI. " The Old Mouth and the Olfactory Organ : the Meaning

of the First Nei*ve," vol. xxxiv., p. 514.
19oo. „ VII. " The Evidence of Prosomatic Appendages in Ammocoetes,

as given by the Course and Distribution of the Trigeminal

Nerve," vol. xxxiv., p. 537.

1900. .. VIII. "The Pakeontological Evidence: Ammocoetes a Cepha-

laspid," vol. xxxiv., p. 562.

1901. .. IX. "The Origin of the Optic Apparatus: the Meaning of the

Optic Nerves," vol. xxxv., p. 224.

INTRODUCTION

1902. Part X. " The Origin of the Auditory Organ : the Meaning- of the

Vlllth Cranial Nerve," vol. xxxvi., p. 164.

1903. ., XI. '" The Origin of the Vertebrate Body-cavity and Excretory

Organs : the Meaning of the Somites of the Trunk and
of the Ductless Glands," vol. xxxvii., p. 168.

1905. .. XII. " The Principles of Embryology," vol. xxxix., p. 371.

1906. .. XIII. " The Origin of the Notochord and Alimentary Canal,"

vol. xl., p. 305.

CHAPTER I

THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM

Theories of the origin of vertebrates. — Importance of the central nervous
system. — Evolution of tissues. — Evidence of Palaeontology. — Reasons for
choosing- Animocoetes rather than Amphioxus. — Importance of larval forms.
— Comparison of the vertebrate and arthropod central nervous systems. —
Antagonism between cephalization and alimentation. — Life-history of
lamprey : not a degenerate animal.— Brain of Animocoetes compared with
brain of arthropod. — Summary.

At the present time it is no longer a debatable question whether or
no Evolution has taken place. Since the time of Darwin the accu-
mulation of facts in its support has been so overwhelming that all
zoologists look upon this question as settled, and desire now to find
out the manner in which such evolution has taken place. Here two
problems offer themselves for investigation, which can be and are
treated separately — the one dealing with the question of those laws
of heredity and variation which have brought about in the past and
are still causing in the present the evolution of living beings, i.e. the
causes of evolution ; the other concerned with the relationship of
animals, or groups of animals, rather than with the causes which
have brought about such relationship, i.e. the sequence of evolution.

It is the latter problem with which this book deals, and, indeed,
not with the whole question at all, but only with that part of it
which concerns the origin of vertebrates.

This problem of the sequence of evolution is of a twofold character :
first, the finding out of the steps by which the higher forms in
any one group of animals have been evolved from the lower ; and
secondly, the evolution of the group itself from a lower group.

In any classification of the animal kingdom, it is clear that large
groups of animals exist which have so many common characteristics
as to necessitate their being placed in one larger group or kingdom ;

THE EVIDENCE OE THE CENTRAL NERVOUS SYSTEM 9

thus zoologists are able to speak definitely of the Vertebrata, Arthro-
poda, Annelida, Echinodermata, Porifera, Ccelenterata, Mollusca,
etc. In each of these groups affinities can be traced between the
members, so that it is possible to speak of the progress from lower
to higher members of the group, and it is conceivable, given time to
work out the details, that the natural relationships between the
members of the whole group will ultimately be discovered.

Thus no one can doubt that a sequence of the kind has taken
place in the Vertebrata as we trace the progress from the lowest fishes
to man, and already the discoveries of palaeontology and anatomy
give us a distinct clue to the sequence from fish to amphibian, from
amphibian to reptile, from reptile to mammal on the one hand, and
to bird on the other. That the different members of the vertebrate
group are related to each other in orderly sequence is no longer a
matter of doubt ; the connected problems are matters of detail, the
solution of which is certain sooner or later. The same may be said
of the members of any of the other great natural groups, such as
the Arthropoda, the Annelida, the Echinodermata, etc.

It is different, however, when an attempt is made to connect
two of the main divisions themselves. It is true enough that there
is every reason to believe that the arthropod group has been evolved
from the segmented annelid, and so the whole of the segmented
invertebrates may be looked on as forming one big division, the
Appendiculata, all the members of which will some day be arranged
in orderly sequence, but the same feeling of certainty does not exist
in other cases.

In the very case of the origin of the Appendiculata we are con-
fronted with one of the large problems of evolution — the origin of
segmented from non-segmented animals — the solution of which is not
yet known.

Theories of the Origin of Vertebrates.

The other large problem, perhaps the most important of all, is the
question of the relationship of the great kingdom of the Vertebrata :
from what invertebrate group did the vertebrate arise ?

The great difficulty which presents itself in attempting a solution
of this question is not so much, as used to be thought, the difficulty
of deriving a group of animals possessing an internal bony and

THE ORIGIN OF VERTEBRATES

cartilaginous skeleton from a group possessing an external skeleton
of a calcareous or chitinous nature, but rather the difficulty caused by
the fundamental difference of arrangement of the important internal
organs, especially the relative positions of the central nervous system
and the digestive tube.

Now, if we take a broad and comprehensive view of the inver-
tebrate kingdom, without arguing out each separate case, we find that

Fig. 1. — Arrangement of Organs in the Vertebrate (A) and Arthropod (B)'
Al, gut; IT, heart; C.N.S., central nervous system; V, ventral side; D, dorsal side.

it bears strongly the stamp of a general plan of evolution derived
from a co;lenterate animal, whose central nervous system formed a
ring surrounding the mouth. Then when the radial symmetry was
given up, and an elongated, bilateral, segmented form evolved, the
central nervous system also became elongated and segmented, but,
owing to its derivation from an oral ring, it still surrounded the
mouth-tube, or oesophagus, and thus in its highest forms is divided into
supra- oesophageal and infra-oesophageal nervous masses. These latter

THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM II

nervous masses are of necessity ventral to the digestive tube, because
the mouth of the ccelenterate is on the ventral side. The striking
characteristic, then, of the invertebrate kingdom is the situation of a
large portion of the central nervous system ventrally to the alimentary
canal and the piercing of the nervous system by a tube — the oeso-
phagus— leading from the mouth to the alimentary canal. The
equally striking characteristic of the vertebrate is the dorsal position
of the central nervous system and the ventral position of the ali-
mentary canal combined with the absence of any piercing of the
central nervous system by the oesophagus.

So fundamentally different is the arrangement of the important
organs in the two groups that it might well give rise to a feeling of
despair of ever hoping to solve the problem of the Origin of Verte-
brates; and, to my mind, this is the prevalent feeling among
morphologists at the present time. Two attempts at solution have
been made. The one is associated with the name of Geoffrey St.
Hilaire, and is based on the supposition that the vertebrate has
arisen from the invertebrate by turning over on its back, swimming
in this position, and so gradually converting an originally dorsal
surface into a ventral one, and vice versa ; at the same time, a new
mouth is supposed to have been formed on the new ventral side,
which opened directly into the alimentary canal, while the old
mouth, which had now become dorsal, was obliterated.

The other attempt at solution is of much more recent date, and is
especially associated with the name of Bateson. It supposes that
bilaterally symmetrical, elongated, segmented animals were formed
from the very first in two distinct ways. In the one case the diges-
tive tube pierced the central nervous system, and was situated dorsally
to its main mass. In the other case the segmented central nervous
system was situated from the first dorsally to the alimentary canal,
and was not pierced by it. In the first case the highest result of
evolution led to the Arthropoda ; in the second case to the Vertebrata.

Neither of these views is based on evidence so strong as to cause
universal acceptance. The great difficulty in the way of accepting
the second alternative is the complete absence of any evidence, either
among animals living on the earth at the present day or among those
known to have existed in the past, of any such chain of intermediate
animal forms as must, on this hypothesis, have existed in order to
link together the lower forms of life with the vertebrates.

THE ORIGIN OF VERTE B RATES

It has been supposed that the Tunicata and the Enteropneusta
{Balanoglossus) (Fig. '2) are members of this missing chain, and that

in Amphioxus the ver-
tebrate approaches in
organization to these
low invertebrate forms.
The timicates, indeed,
are looked upon as de-
generate members of an
early vertebrate stock,
which may give help in
picturing the nature of
the vertebrate ancestor
but are not themselves

in the direct line of
Fig. 2.— Larval Balanoglossus (from the Royal

Natural History). descent. Balanoglossus

is supposed to have
arisen from the Echinodermata, or at all events to have affinities
with them, so that to fill up the enormous gap between the
Echinodermata and the Vertebrata on this theory there is absolutely
nothing living on the earth except Balanoglossus, Bhabdopleura,
and Cephalodiscus. The characteristics of the vertebrate upon
which this second theory is based are the notochord, the respiratory
character of the anterior part of the alimentary canal, and the tubular
nature of the central nervous system ; it is claimed that in Balano-
glossus the beginnings of a notochord and a tubular central nervous
system are to be found, while the respiratory portion of the gut is
closely comparable to that of Amphioxus.

The strength of the first theory is essentially based on the com-
parison of the vertebrate central nervous system with that of tho
segmented in vertel irate, annelid or arthropod. In the latter the
central nervous system is composed of —

1. The supra-cesophageal ganglia, which give origin to the nerves
of the eyes and antennules, i.e. to the optic and olfactory nerves,
for the first pair of antenna? are olfactory in function. These are
connected with the infra-cesophageal ganglia by the oesophageal
commissures which encircle the oesophagus.

2. The infra-cesophageal ganglia and the two chains of ventral
ganglia, which are segmentally-arranged sets of ganglia. Of these,

THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM I *

each pair gives rise to the nerves of its own segment, and these
nerves are not nerves of special sense as are the supra-cesophageal
nerves, but motor and sensory to the segment ; nerves by the agency
of which food is taken in and masticated, respiration is effected, and
the animal moves from place to place.

In the vertebrate the central nervous system consists of —

1. The brain proper, from which arise only the olfactory and optic

nerves.

DORSAL

Spinal canal

Neureateric canal

H JMI | II p r'

Spinal Cord « Seomenlal Nerves

U/un^tulum VENTRAL

DORSAL

• ■■'aiopKaju. VENTRAL

Fig. 3. — Vertebrate Central Nervous System compared with the Central
Nervous System and Alimentary Canal of the Arthropod.

A. Vertebrate central nervous system. 8. Inf. Br., supra-infundibular brain;
I. Inf. Br., infra-infundibular brain and cranial segmental nerves; C.Q., corpora
quadrigemina ; Cb., cerebellum; C.C., crura cerebri; C.S., corpus striatum; Fn..
pineal gland.

B. Invertebrate central nervous system. <S'. (27s. G., supra-cesophageal ganglia ;
I. (Es. G., infra-cesopbageal ganglia; QSs. Com., oesophageal commissures.

2. The region of the mid-brain, medulla oblongata, and spinal
cord ; from these arises a series of nerves segmentally arranged,
which, as in the invertebrate, gives origin to the nerves governing
mastication, respiration, and locomotion.

Further, the vertebrate central nervous system possesses the
peculiarity, found nowhere else, of being tubular, and the tube is
of a striking character. In the spinal region it is a small, simple
canal of uniform calibre, which at the front end dilates to form the
ventricles of the region of the brain. From that part of this dilated

14 THE ORIGIN OF VERTEBRATES

portion, known as the third ventricle, a narrow tube passes to the
ventral surface of the brain. This tube is called the infundibulum,
and, extraordinary to relate, lies just anteriorly to the exits of the
third cranial or oculomotor nerves ; in other words, it marks the
termination of the series of spinal and cranial segmental nerves.
Further, on each side of this infundibular tube are lying the two
thick masses of the crura cerebri, the strands of fibres which connect
the higher brain-region proper with the lower region of the medulla
oblongata and spinal cord. Not only, then, are the nerve-masses
in the two systems exactly comparable, but in the very place where
the oesophageal tube is found in the invertebrate, the infundibular
tube exists in the vertebrate, so that if the words infundibular and
oesophageal are taken to be interchangable, then in every respect
the two central nervous systems are comparable. The brain proper
of the vertebrate, with its olfactory and optic nerves, becomes the
direct descendant of the supra-cesophageal ganglia ; the crura cerebri
become the oesophageal commissures, and the cranial and spinal
segmental nerves are respectively the nerves belonging to the infra-
cesophageal and ventral chain of ganglia.

This overwhelmingly strong evidence has always pointed directly
to the origin of the vertebrate from some form among the segmented
group of invertebrates, annelid or arthropod, in which the original
oesophagus had become converted into the infundibulum, and a new
mouth formed. So far, the position of this school of anatomists was
extremely sound, for it is impossible to dispute the facts on which
it is based. Still, however, the fact remained that the gut of the
vertebrate lies ventrally to the nervous system, while that of the
invertebrate lies dorsally ; consequently, since the infundibulum was
in the position of the invertebrate oesophagus, it must originally have
entered into the gut, and since the vertebrate gut was lying ventrally
to it, it could only have opened into that gut in the invertebrate stage
by the shifting of dorsal and ventral surfaces. From this argument
it followed that the remains of the original mouth into which the in-
fundibulum, i.e. oesophagus, opened were to be sought for on the dorsal
side of the vertebrate brain. Here in all vertebrates there are two
spots where the roof of the brain is very thin, the one in the region of
the pineal body, and the other constituting the roof of the fourth ven-
tricle. Both of these places have had their advocates as the position of
the old mouth, the former being upheld by Owen, the latter by Dohrn.

THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM 1 5

The discovery that the pineal body was originally an eye, or,
rather, a pair of eyes, has perhaps more than anything else proved
the impossibility of accepting this reversal of surfaces as an explana-
tion of the genesis of the vertebrate from the annelid group. For
whereas a pair of eyes close to the mid-dorsal line is not only likely
enough, but is actually found to exist among large numbers of
arthropods, both living and extinct, a pair of eyes situated close
to the mid-ventral line near the mouth is not only unheard of in
nature, but so improbable as to render impossible the theory which
necessitates such a position.

Yet this very discovery gives the strongest possible additional
support to the close identity in the plan of the central nervous
system of vertebrate and appendiculate.

A truly paradoxical situation ! The very discovery which may
almost be said to prove the truth of the hypothesis, is the very one
which has done most to discredit it, because in the minds of its
authors the only possible solution of the transition from the one
group to the other was by means of the reversal of surfaces.

Still, as already said, even if the theory advanced to explain the
facts be discredited, the facts remain the same ; and still to this day
an explanation is required as to why such extraordinary resemblances
should exist between the two nervous systems, unless there is a
genetic connection between the two groups of animals. An ex-
planation may still be fouud, and must be diligently sought for,
which shall take into account the strong evidence of this relation-
ship between the two groups, and yet not necessitate any reversal
of surfaces. It is the object of this book to consider the possibility
of such an explanation.

What are the lines of investigation most likely to meet with
success ? Is it possible to lay down any laws of evolution ? It
is instructive to consider the nature of the investigations which
have led to the two theories just mentioned, for the fundamental
starting-point is remarkably different in the two cases. The one
theory is based upon the study of the vertebrate itself, and especially
of its central nervous system, and its supporters and upholders have
been and are essentially anatomists, whose chief study is that of
vertebrate and human anatomy. The other theory is based upon the
study of the invertebrate, and consists especially of an attempt to
find in the invertebrate some structure resembling a notochord, such

1 6 THE ORIGIN OF VERTEBRATES

organ being considered by them as the great characteristic of the
vertebrate ; indeed, so much is this the case, that a large number
of zoologists speak now of Chordata rather than of Vertebrata, and
in order to emphasize their position follow Bateson, and speak of the
Tunicata as Uro-chordata, of Amphioxus as Cephalo-chordata, of the
Enteropneusta as Hemi-chordata, and even of Actinotrocha (to use
Masterman's term), as Diplo-chordata.

The upholders of this theory lay no stress on the nature of the
central nervous system in vertebrates, they are essentially zoologists
who have made a special study of the invertebrate rather than of
the vertebrate.

Of these two methods of investigating the problem, it must be
conceded that the former is more likely to give reliable results.
By putting the vertebrate to the question in every possible way, by
studying its anatomy and physiology, both gross and minute, by
inquiring into its past history, we can reasonably hope to get a
clue to its origin, but by no amount of investigation can we tell
with any certainty what will be its future fate ; we can only guess
and prophesy in an uncertain and hesitating manner. So it must be
with any theory of the origin of vertebrates, based on the study of
one or other invertebrate group. Such theory must partake rather
of the nature of prophecy than of deduction, and can only be placed
on a firm basis when it so happens that the investigation of the
vertebrate points irresistibly to its origin from the same group ; in
fact, " never prophesy unless you know."

The first principle, then, I would lay down is this : In order to
find out the origin of vertebrates, inquire, in the first place, of the
vertebrate itself.

Impoetance of the Central Nervous System.

Does the history of evolution pick out any particular organ or
group of organs as more necessary than another for upward progress ?
If so, it is upon that organ or group of organs that special stress must
be laid.

Since Darwin wrote the " Origin of Species," and laid down that
the law of the ' survival of the fittest ' is the factor upon which evolu-
tion depends, it has gradually dawned upon the scientific mind that
' the iittest ' may be produced in two diametrically opposite ways :

THE EVIDENCE OE THE CENTRAL NERVOUS SYSTEM 1 7

either by progress upwards to a superior form, or by degeneration to
a lower type of animal. The principle of degeneration as a factor
in the formation of groups of animals, which are thereby enabled
to survive, is nowadays universally admitted. The most striking
example is to be found in the widely distributed group of Tunicata,
which live, in numbers of instances, a sedentary life upon the rocks,
have the appearance of very low forms of animal life, propagate
by budding, have lost all the characteristics of higher forms, and
yet are considered to be derived from an original vertebrate stock.
Such degenerate forms remain degenerate, and are never known to
regenerate and again to reach the higher stage of evolution from
which they arose. Such forms are of considerable interest, but
cannot help, except negatively, to decide what factor is especially
important for upward progress.

At the head of the animal race at the present day stands man,
and in mankind itself some races are recognized as higher than others.
Such recognition is given essentially on account of their greater
brain-power, and without doubt the great characteristic which puts
man at the head is the development of his central nervous system,
especially of the region of the brain. Not only is this point most
manifest in distinguishing man from the lower animals, but it applies
to the latter as well. By the amount of convolution of the brain,
the amount of grey matter in the cerebral hemispheres, the enlarge-
ment and increasing complexity of the higher parts of the central
nervous system, the anthropoid apes are differentiated from the lower
forms, and the higher mammals from the lower. In the recent work
of Elliot Smith, and of Edinger, most conclusive proof is given that
the upward progress in the vertebrate phylum is correlated with the
increase of brain-power, and the latter writer shows how steady and
remarkable is the increase in substance and in complexity of the
brain-region as we pass from the fishes, through the amphibians and
reptiles, to the birds and mammals.

The study of the forms which lived on the earth in past ages con-
firms and emphasizes this conclusion, for it is most striking to see
how small is the cranium among the gigantic Dinosaurs ; how in the
great reptilian age the denizens of the earth were far inferior in brain-
power to the lords of creation in after-times.

What applies to the vertebrate phylum applies also to the inver-
tebrate groups. Here also an upward progress is recognized as we

1 8 THE ORIGIN OF VERTEBRATES

pass from the sponges to the arthropods — a progress which is mani-
fested, first by the concentration of nervous material to form a central
nervous system, and then by the increase in substance and complexity
of that nervous system to form a higher and a higher type, until the
culmination is reached in the nervous system of the scorpions and
spiders. No upward progress is possible with degeneration of the
central nervous system, and in all those cases where a group owes its
existence to degeneration, the central nervous system takes part in
the degeneration.

This law of the paramount importance of the growth of the central
nervous system for all upward progress in the evolution of animals
receives confirmation from the study of the development of individuals,
especially in those cases where a large portion of the life of the
animal is spent in a larval condition, and then, by a process of trans-
formation, the larva changes into the adult form. Such cases are
well known among Arthropoda, the familiar instance being the change
from the larval caterpillar to the adult imago. Among Vertebrata,
the change from the tadpole to the frog, from the larval form of
the lamprey (Ammocwtes) to the adult form (Petromyzon), are well-
known instances. In all such cases the larva shows signs of having
attained a certain stage in evolution, and then a remarkable trans-
formation takes place, with the result that an adult animal emerges,
whose organization reaches a higher stage of evolution than that of
the larva.

This transformation process is characterized by a very great
destruction of the larval tissues and a subsequent formation of new
adult tissues. Most extensive is the destruction in the caterpillar
and in the larval lamprey. But one organ never shares in this process
of histolysis, and that is the central nervous system ; amidst the
ruins of the larva it remains, leading and directing the process of
re-formation. In the Arthropoda, the larval alimentary canal may
be entirely destroyed and eaten up by phagocytes, but the central
nervous system not only remains intact but increases in size, and by
the concentration and cephalization of its infra-cesophageal ganglia
forms in the adult a central nervous system of a higher type than
that of the larva.

So, too, in the transformation of the lamprey, there is not the
slightest trace of any destruction in the central nervous system, but
simply a development and increase in nervous material, which

THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM 1 9

results in the formation of a brain region more like that of the higher
vertebrates than exists in Animocoetes.

In these cases the development is upward — the adult form is of a
higher type than that of the larva. It is, however, possible for the
reverse to occur, so that the individual development leads to degene-
ration, not to a higher type. Instances are seen in the Tunicata, and
in various parasitic arthropod forms, such as Lernaea, etc. In these
cases, the transformation from the larval to the adult form leads to
degradation, and in this degradation the central nervous system is
always involved.

It is perhaps a truism to state that upward progress is necessarily
accompanied by increased development of the central nervous system ;
but it is necessary to lay special stress upon the importance of the
central nervous system in all problems of evolution, because there is,
in my opinion, a tendency at the present time to ignore this factor to
too great an extent.

The law of progress is this — The race is not to the swift, nor to
the strong, but to the wise.

This law carries with it the necessary corollary that the imme-
diate ancestor of the vertebrate must have had a central nervous
system nearly approaching that of the lowest undegenerated verte-
brate. Among all the animals living on the earth at the present
time, the highest invertebrate group, the Arthropoda, possesses
a central nervous system most closely resembling that of the
vertebrate.

The law, then, of the paramount importance of a steady develop-
ment of the central nervous system for the upward progress of the
animal kingdom, points directly to the arthropod as the most probable
ancestor of the vertebrate.

Evolution of Tissues.

In the whole scheme of evolution we can recognize, not only an
upward progress in the organization of the animal as a whole, but
also a distinct advance in the structure of the tissues composing an
individual, which accompanies that upward progress. Thus it is
possible to speak of an evolution of the supporting tissues from the
simplest form of connective tissue up to cartilage and thence to bone;
of the contractile tissues, from the simplest contractile protoplasm

20 THE ORIGIN OF VERTEBRATES

to unstriped muscle, and thence to the highest forms of striated
muscle ; of the nervous connecting strands, from undifferentiated to
fine strands, then to thicker, more separated ones, resembling non-
medullated fibres, and finally to well-differentiated separate fibres,
each enclosed in a medullated sheath.

In the connective tissue group, bone is confined to the vertebrates,
cartilage is found among invertebrates, and the closest resemblance
to vertebrate embryonic or parenchymatous cartilage is found in the
cartilage of Limulus. Also, as Gegenbaur has pointed out, Limulus,
more than any other invertebrate, possesses a fibrous connective
tissue resembling that of vertebrates.

In the muscular group, Biedermann, who has made a special
study of the physiology of striated muscle, says that among inver-
tebrates the striated muscle of the arthropod group resembles most
closely that of the vertebrate.

In the nervous group the resemblance between the nerve-fibres
of Limulus and Ammoccetes, both of which are devoid of any marked
medullary sheath, is very apparent, and Eetzius points out that the
only evidence of medullation, so characteristic of the vertebrates, is
found in a species of prawn (Palamion). In all these cases the
nearest resemblance to the vertebrate tissues is to be found in the
arthropod.

The Evidence of Paleontology.

Perhaps the most important of all the clues likely to help in the
solution of the origin of vertebrates is that afforded by Geology, for
although the geological record is admittedly so imperfect that we
can never hope by its means alone to link together the animals at
present in existence, yet it does undoubtedly point to a sequence in
the evolution of animal forms, and gives valuable information as to
the nature of such sequence. In different groups of animals there
are times when the group can be spoken of as having attained its
most flourishing period. During these geological epochs the dis-
tribution of the group was universal, the numbers were very great,
the number of species was at the maximum, and some of them had
attained a maximal size. Such races were at that time dominant,
and the struggle for existence was essentially among members of the
same group. At the present time the dominant race is man, and the

THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM 21

struggle for existence is essentially between the members of that
race, and not between them and any inferior race.

The effect of such conditions is, as Darwin has pointed out, to
cause great variation in that group ; in consequence of that variation
and that dominance the evolution of the next higher group is brought
about from some member of the dominant group. Thus the present
age is the outcome of the Tertiary period, a time when giant mammals
roamed the earth and left as their successors the mammals of the
present day ; a time of dominance of quadruped mammals ; a time
of which the period of maximum development is long past, and we
now see how the dominance of the biped mammal, man, is accom-
panied by the rapid diminution and approaching extermination of
the larger mammals. No question can possibly arise as to the im-
mediate ancestor of the biped mammal ; he undoubtedly arose from
one of the dominant quadrupedal mammals.

Passing along to the next evidence of the rocks, we find an age of
reptiles in the Mesozoic period. Here, again, the number and
variety is most striking ; here, again, the size is enormous in com-
parison with that of the present-day members of the group. This
was the dominant race at the time when the birds and mammals
first appeared on the earth, and anatomists recognize in these extinct
reptilian forms two types ; the one bird-like, the other more mamma-
lian in character. From some members of the former group birds
are supposed to have been evolved, and mammals from members of
the other group. There is no question of their origin directly from
lower fish-like forms ; the time of their appearance on the earth,
their structure, all point irresistibly to the same conclusion as we
have arrived at from the consideration of the origin of the biped
from the quadruped mammal, viz. that birds and mammals arose, in
consequence of the struggle for existence, from some members of the
reptilian race which at that time was the dominant one on earth.

Passing down the geological record, we find that when the reptiles
first appear in the Carboniferous age there is abundant evidence of
the existence of numbers of amphibian forms. At this time the
giant Labyrinthodonts flourished. Here among the swamps and
marshes of the coal-period the prevalent vertebrate was amphibian
in structure. Their variety and number were very great, and at that
period they attained their greatest size. Here, again, from the
geological record we draw the same conclusion as before, that the

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THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM 23

reptiles arose from the race which was then predominant on the earth
— the Amphibia.

Again, another point of great interest is seen here, and that is
that these Labyrinthodonts, as Huxley has pointed out, possess
characters which bring them more closely than the amphibians of
the present day into connection with the fishes; and further, the
fish-like characters they possessed are those of the Ganoids, the
Marsipobranchs, the Dipnoans, and the Elasmobranchs, rather than
of the Teleosteans.

Now, it is a striking fact that the ancient fishes at the time when
the amphibians appeared had not reached the teleostean stage. The
ganoids and elasmobranchs swarmed in the waters of the Devonian
and Carboniferous times. Dipnoans and marsipobranchs were there,
too, in all probability, but teleosteans do not appear until the
Mesozoic period. The very kinds of fish, then, which swarmed in
the seas at that time, and were the predominant race before the
Carboniferous epoch, are those to which the amphibians at their first
appearance show the closest affinity. Here, again, the same law
appears ; from the predominant race at the time, the next higher
race arose, and arose by a most striking modification, which was the
consequence of altering the medium in which it lived. By coming
out of the water and living on the land, or, rather, being able to live
partly on land and partly in the water, by the acquisition of air-
breathing respiratory organs or lungs in addition to, and instead of,
water-breathing organs or gills, the amphibian not only arose from
the fish, but made an entirely new departure in the sequence of
progressive forms.

This was a most momentous step in the history of evolution —
one fraught with mighty consequences and full of most important
suggestions.

From this time onwards the struggle for existence by which
upward progress ensued took place on the land, not in the sea, and,
as has been pointed out, led to the evolution of reptiles from am-
phibians, birds and quadrupedal mammals from reptiles, and man
from quadrupeds. In the sea the fishes were left to multiply and
struggle among themselves, their only opponents being the giant
cephalopods, which themselves had been evolved from a continual
succession of the Mollusca. For this reason the struggle for existence
between the fishes and the higher race evolved from them did not

24 THE ORIGIN OF VERTEBRATES

take place until some members of that higher race took again to the
water, and so competed with the fish-tribe in their own element.

Another most important conclusion to be derived from the
uprising of the Amphibia is that at that time there was no race
of animals living on the land which had a chance against them. No
race of land-living animals had been evolved whose organization
enabled them to compete with and overcome these intruders from
the sea in the struggle for existence. For this reason that the
whole land was their own, and no serious competition could arise
from their congeners, the fish, they took possession of it, and increased
mightily in size ; losing more and more the habit of going into the
water, becoming more and more truly terrestrial animals. Hence-
forth, then, in trying to find out the sequence of evolution, we must
leave the land and examine the nature of the animals living in the
sea ; the air-breathing animals which lived on the land in the Upper
Silurian and Devonian times cannot have reached a stage of organi-
zation comparable with that of the fishes, seeing how easily the
amphibians became dominant.

We arrive, then, at the conclusion that the ancestors of the fishes
must have lived in the sea, and applying still the same principles
that have held good up to this time, the ancestors of the fishes must
have arisen from some member of the race predominant at the time
when they first appeared, and also the earliest fishes must have much
more closely resembled the ancestral form than those found in later
times or at the present day.

What, then, is the record of the rocks at the time of the first
appearance of fish-like forms ? What kind of fishes were they, and
what was the predominant race at the time ?

We have now reached the Upper Silurian and Lower Devonian
times, and most instructive and suggestive is the revelation of the
rocks. Here, when the first vertebrates appeared, the sea was peopled
with corals, brachiopods, .early forms of cephalopods, and other in-
vertebrates ; but, above all, with the great tribe of trilobites (Fig. 6)
and their successors. From the trilobites arose, as evidenced by
their larval form, the king-crab group, called the Xiphosura (Fig. 5).
Closely connected with them, and forming intermediate stages
between trilobites and king-crabs, numerous forms have been dis-
covered, known as Belinurus, Prestwichia, Hemiaspis, Bunodes, etc.
(Fig. 5 and Fig. 12). From them also arose the most striking group

THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM 25

of animals which existed at this period — the giant sea-scorpions, or
Gigantostraca. This group was closely associated with the king-
crabs, and the two groups together are classified under the title
Merostomata.

The appearance of these sea-scorpions is given iti Figs. 7 and 8,
representing Stylonurus, Slimonia, Pterygotus, Eurypterus. They

Fig. 5 (from H. Woodward). — 1. Limulus polyphemus (dorsal aspect). 2. Lunulas,
young, in trilobitc stage. 3. Prestwichia rotundata. 4. Prestivichia Birtwelli.
5. Hemiaspis limuloides. 6. Pseudoniscus acitlcatus.

must have been in those days the tyrants of the deep, for specimens
of Pterygotus have been found over six feet in length.

At this time, then, by every criterion hitherto used, by the
multitude of species, by the size of individual species, which at this
period reached the maximum, by their subsequent decay and final
extinction, we must conclude that these forms were in their zenith,
that the predominant race at this time was to be found in this group
of arthropods. Just previously, the sea swarmed with trilobites, and
right into the period when the Gigantostraca flourished, the tril obites

THE ORIGIN OF VERTEBRATES

are still found of countless forms, of great difference in size. The
whole period may be spoken of as the great trilobite age, just as the
Tertiary times form the mammalian age, the Mesozoic times the
reptilian age, etc. From the trilobites the Gigantostraca and
Xiphosura arose, as evidenced by the embryology of Limulus, and,
therefore, in the term trilobite age would be included the whole of
those peculiar forms which are classified by the names Trilobita,

Fig. 6.— A Trilobite (Dalma-
tites) (after Pictet). Dorsal

Fig. 7. — Euryplerus remises (after
Nieskowski). Dorsal view.

view.

Gigantostraca, Xiphosura, etc. Of all these the only member alive
at the present time is Limulus, or the King-Crab.

As, however, the term ' trilobite ' does not include the members
of the king-crab or sea-scorpion groups, it is advisable to use some
other term to represent the whole group. They cannot be called
crustaceans or arachnids, for in all probability they gave origin to
both ; the nearest approach to the Trilobite stage of development at
the present time is to be found perhaps in Branchipus (Fig. 10) and
Apus (Fig. 9), just as the nearest approach to the Eurypterid

THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM 27

form is Limulus. Crustaceans such as crabs and lobsters are of
much later origin, and do not occur in any quantity until the late

Fig. 8. — A, Pterygotus Osiliensis (from Schmidt). B, Stylonurus Logani (from
Woodward). C, Slimonia acuminata (from Woodward).

Mesozoic period. The earliest found, a kind of prawn, occurs in the
Carboniferous age.

Korschelt and Heider have accordingly suggested the name
Palceostraca for this whole group, and Protostraca for the still earlier

THE ORIGIN OF VERTEBRATES

arthropod-like animals which gave origin to the trilobites themselves.
This name I shall adopt, and speak, therefore, of the Palasostraca as

the dominant race at the time when
vertebrates first appeared.

If, then, there is no break in the
law of evolution here, the race which
was predominant at the time when
the vertebrate first appeared must
have been that from which the first
fishes arose, and these fishes must
have resembled, not the crustacean
proper, or the arachnid proper, but a
member of the palreostracan group.
Moreover, just as the Labyrinthodonts
show special affinities to the fishes
which were then living, so we should
expect that the forms of the earliest
fish would resemble the arthropodan
type dominant at the time more
closely than the fish of a later era.

At first sight it seems too great
an absurdity even to imagine the
possibility of any genetic connection between a fish and an arthropod,
for to the mind's eye there arises immediately the picture of a
salmon or a shark and a lobster or a spider. So different in appear-

Fig. 9. — Apus (from the Royal
Natural History). Dorsal view.

Fig. 10.— Branchipus stagnalis. (From Claus.)

ance are the two groups of animals, so different their methods of
locomotion, that it is apparently only an inmate of a lunatic asylum

THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM 29

who could possibly suggest such a connection. Much more likely
is it that a fish-like form should have been developed out of a smooth,
wrio-a-lincr worm-like animal, and it is therefore to the annelids that
the upholders of the theory of the reversal of surfaces look for the
ancestor of the vertebrate.

We must endeavour to dismiss from our imagination such forms
as the salmon and shark as representatives of the fish-tribe, and the
lobster and spider of the arthropods, and try to picture the kind of
animals living in the seas in the early Devonian and Upper Silurian
times, and then we find, to our surprise, that instead of the contrast
between fishes and arthropods being so striking as to make any
comparison between the two seem an absurdity, the difficulty in the
last century, and even now, is to decide in many cases whether a
fossil is an arthropod or a fish.

I have shown what kind of animal the palaeostracan was like.
What information is there of the nature of the earliest vertebrate ?

The most ancient fishes hitherto discovered have been classified
by Lankester and Smith Woodward into the three orders, Hetero-
straci, Osteostraci, and Antiarcha. Of these the Heterostraci contain
the genera Pteraspis and Cyathaspis, and are the very earliest
vertebrates yet discovered, being found in the Lower Silurian. The
Osteostraci are divided into the Cephalaspidae, Tremataspida?, etc.,
and are found in the Upper Silurian and Devonian beds. The
Antiarcha, comprising Pterichthys and Bothriolepis, belong to the
Devonian and are not found in Silurian deposits. This, then, is the
order of their appearance— Pteraspis, Cephalaspis, and Pterichthys.

In none of these families is there any resemblance to an ordinary
fish. In no case is there any sign of vertebra? or of jaws. They, like
the lampreys, were all agnathostomatous. Strange indeed is their
appearance, and it is no wonder that there should have been a
difficulty in deciding whether they were fish or arthropod. Their great
characteristic is their buckler- plated cephalic shield, especially con-
spicuous on the dorsal side of the head. Figs. 11, 14, 15, 16, give
the dorsal shields of Pteraspis, Auchenaspis, Pterichthys, and
Bothriolepis.

In 1904, Drevermann discovered a mass of Pteraspis Dunensis
embedded in a single stone, showing the same kind of head-shield
as P. rostrcda, but the rostrum was longer and the spine at the
extremity of the head-shield much longer and more conspicuous.

THE ORIGIN OF VERTEBRATES

Fig. 11. — Ptcraspis duncnsis (from Drevermann). Dorsal view of body and spine
on the right side. Head-end, showing long rostrum on the left side.

Fig. 12. — Bunodes lunula. (From
Schmidt.)

Fig. 13. — Auclicnaspis (Tkyestes) verru-
cosus, natural size. (From Woodward.)

THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM 3 1

The whole shape of the animal as seen in this photograph recalls the
shape of a Hemiaspid rather than of a fish. It is, then, natural
enough for the earlier observers to have looked upon such a fossil as
related to an arthropod rather than a fish.

In Figs. 12 and 13 I have placed side by side two Silurian fossils
which are found in the same geological horizon. They are both life
size and possess a general similarity of appearance, yet the one is a

Fig. 14.— Dorsal Head-shield of Thy
estes (Auchenaspis) verrucosus. (From
Rohon.)

Fro., narial opening; i.e., lateral eyes; gl.,
glabellum or plate over brain; Occ, oc-
cipital region.

Fig. 15. — Ptcricthys.

Cephalaspidian fish known by the name of Auchenasjris or Tlu/estes
verrucosa, the other a Palreostracan called Bunodes lunula.

In a later chapter I propose to discuss the peculiarities and the
nature of the head-shields of these earliest fishes, in connection with
the question of the affinities of the animals which bore them. At
this point of my argument I want simply to draw attention to the
undoubted fact of the striking similarity in appearance between the

THE ORIGIN OF VERTEBRATES

earliest fishes and members of the Palaaostraca, the dominant race of
arthropods which swarmed in the sea at the time : a similarity which
could never have been suspected by any amount of investigation

Fig. 16. — Bothriolepis. (After Patten.)
An., position of anus.

among living forms, but is immediately revealed when the ages
themselves are questioned.

I have not reproduced any of the attempted restorations of these
old forms, as usually given in the text-books, because all such restora-
tions possess a large element of fancy, due to the personal bias of the
observer. I have put in Eohon's idea of the general shape of Tre-
mataspis (Fig. 17) in order to draw attention to the lamprey-like
appearance of the fish according to his researches (cf. Fig. 18).

Fig. 17. — Kestoration of Tremataspis. (After Kohon, slightly modified.)

*&s(

Fig. 18. — Ammoecetes.

The argument, then, from geology, like that from comparative
anatomy and from the consideration of the importance of the central
nervous system in the upward development of the animal race, not
only points directly to the arthropod group as the ancestor of the

THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM 33

vertebrate, but also to a distinct ancient type of arthropod, the
Palseostracan, the only living example of which is the King- Crab or
Limulus ; while the nearest approach to the trilobite group among
living arthropods are Branchipus and Apus. It follows, therefore, that
for the following up of this clue, Limulus especially must be taken into
consideration, while Branchipus and Apus are always to be kept in mind.

Ammoccetes rather than Amphioxus is the Best Subject for

Investigation.

It is not, however, Limulus that must be investigated in the first
instance, but the vertebrate itself; for it can never be insisted on too
often that in the vertebrate itself its past history will be found, but
that Limulus cannot reveal the future of its race. What vertebrate
must be chosen for investigation ? Reasons have been given why
oUr attention should be fixed upon the king-crab rather than on the
lobster on the invertebrate side ; what is the most likely animal on
the vertebrate side ?

From the evidence already given it is manifest that the earliest
mammal belonged to the lowest group of mammals ; that the birds
on their first appearance presented reptilian characteristics, that the
earliest reptiles belonged to a low type of reptile, that the amphibians
at their first appearance were nearer in type to the fishes than were
the later forms. As each of these groups advances in number and
power, specialization takes place in it, and the latest developed
members become further and further removed in type from the
earliest. So also it must have been with the origin of fishes : here
too, in the quest for information as to the structure and nature of
the first-formed fishes, we must look to the lowest rather than to
the highest living members of the group.

The lowest fish-like animal at present living is Amphioxus, and
on this ground it is argued that the original vertebrate must have
approached in organization to that of Amphioxus ; it is upon the
comparison between the structure of Amphioxus and that of Balano-
glossus, that the theory of the origin of vertebrates from forms like
the latter animal is based. For my own part, I think that in the
first instance, at all events, Amphioxus should be put on one side,
although of course its structure must always be kept in mind, for

the following reasons

34 THE ORIGIN OF VERTEBRATES

Amphioxus, like the tunicates, does not possess the character-
istics of other vertebrates. In all vertebrates above these forms
the great characteristic is a well-defined brain-region from which
arise nerves to organs of special sense, the eyes and nose. la
Amphioxus no eyes exist, for the pigmented spot at the anterior
extremity of the brain-region is no eye but only a mass of pig-
ment, and the so-called olfactory pit is a very rudimentary and
inferior organ of smell. In connection with the nearly complete
absence of these two most important sense-organs, the most im-
portant part of the central nervous system, the region corresponding
to the cerebral hemispheres, is also nearly completely absent.

Now, the history of the evolution of the central nervous system in
the animal race points directly to its formation as a concentrated
mass of nervous material at the anterior extremity of the body, in
consequence of the formation of special olfactory and visual organs
at that extremity. As already stated, the concentration of nervous
material around the mouth as an oral ring was its beginning. In
connection with this there arose special sense-organs for the guidance
of the animal to its food which took the form of olfactory and optic
organs. With the shifting from the radial to the elongated form
these sense-organs remained at the anterior or mouth-end of the
animal, and owing to their immense importance in the struggle for
existence, that part of the central nervous system with which they
were connected developed more than any other part, became the
leader to which the rest of the nervous system was subservient, and
from that time onwards the development of the brain-region was
inevitably associated with the upward progress of animal life.

To those who believe in Evolution and the Darwinian theory of
the survival of the fittest, it is simply inconceivable that a soft-bodied
animal living in the mud, blind, with a rudimentary brain and rudi-
mentary olfactory organs, such as is postulated when we think of
Balanoglossus and Amphioxus, should hold its own and come victorious
out of the struggle for existence at a time when the sea was peopled
with powerful predaceous scorpion- and crab-like armour-plated
animals possessing a well-developed brain, good eyes and olfactory
organs, and powerful means of locomotion. Wherever in the scale of
animal development Amphioxus may ultimately be placed, it cannot
be looked upon as the type of the earliest formed fishes such as
appeared in Silurian times.

THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM 35

The next lowest group of living fishes is the M arsipobranchii which
include the lampreys and hag-fishes. To these naturally we must turn
for a clue as to the organization of the earliest fish, for here we find
all the characteristics of the vertebrates represented : a well-formed
brain-region, well-developed eyes and nose, cranial nerves directly
comparable with those of other vertebrates, and even the commence-
ment of vertebra?.

Among these forms the lamprey is by far the best for investiga-
tion, not only because it is easily obtainable in large quantities, but
especially because it passes a large portion of its existence in a larval
condition, from which it emerges into the adult state by a wonderful
process of transformation, comparable in extent with the transforma-
tion of the larval caterpillar into the adult imago. So long does the
lamprey live in this free larval condition, and so different is it in
the adult stage, that the older anatomists considered that the two
states were really different species, and gave the name of Am-
rnoccetes branchialis to the larval stage, while the adult form was
called Petromyzon planeri, or Petromyzon fluviatilis.

This long-continued free-living existence in the larval or Am-
moccetes stage makes the lamprey, more than any other type of
lowly organized fish, invaluable for the present investigation, for
throughout the animal kingdom it is recognized that the larval
form approaches nearer to the ancestral type than the adult form,
whether the latter is progressive or degenerate. Not only are the
tissues formed during the stages which are passed through in a
free-living larval form, serviceable tissues comparable to those
of adult life, but also these stages proceed at so much slower a rate
than do those in the embryo in utero or in the egg, as to make
the larval form much more suitable than the embryo for the investi-
gation of ancestral problems. It is true enough that the free life of
the larva may bring about special adaptations which are not of an
ancestral character, as may also occur during the life of the adult ;
but the evidence is very strong that although some of the peculi-
arities of the larva may be due to such ccenogenetic factors, yet on
the whole many of them are due to ancestral characters, which dis-
appear when transformation takes place, and are not found in the
adult.

Thus if it be supposed that the amphibian arose from the fish,
the tadpole presents more resemblance to the fish than the frog. If

36 THE ORIGIN OF VERTEBRATES

it be supposed that the arthropod arose from the segmented worm,
the caterpillar bears out the suggestion better than the adult imago.
If it be supposed that the tunicate arose from a stock allied to the
vertebrate, it is because of the peculiarities of the larva that such a
supposition is entertained. So, too, if it be supposed that the fish
arose from a member of the arthropod group, the larval form of the
fish is most likely to give decisive information on the point.

For all these reasons the lowest form of fish to be investigated,
in the hopes of finding out the nature of the earliest formed fish, is
not Amphioxus, but Ammoccetes, the larval form of the lamprey — a
form which, as I hope to satisfy my reader after perusal of subse-
quent pages, more nearly resembles the ancient Cephalaspidian fishes
than any other living vertebrate.

Comparison of Central Nervous Systems of Vertebrate and
Arthropod without Eeversal of Surfaces.

So far different lines of investigation all point to the origin of the
vertebrate from arthropods, the group of arthropods in question being
now extinct, the nearest living representative being Limulus ; also to
the fact that of the two theories of the origin of vertebrates, that
one which is based on the resemblance between the central nervous
systems of the Vertebrata and the Appendiculata (Arthropoda and
Annelida) is more in accordance with this evidence than the other,
which is based mainly on the supposed possession of a notochord
among certain animals.

How is it, then, that this theory has been discredited and lost
ground ? Simply, I imagine, because it was thought to necessitate
the turning over of the animal. Let us, then, again look at the
nervous system of the vertebrate, and see whether there is any such
necessity.

As previously mentioned, the comparison of the two central
nervous systems showed such close resemblances as to force those
anatomists who supported this theory to the conclusion that the
infundibular tube was in the position of the original oesophagus ;
they therefore looked for the remains of a mouth opening in the
dorsal roof of the brain, but did not attempt to explain the extra-
ordinary fact that the infundibular tube is only a ventral offshoot
from the tube of the central nervous system. Yet this latter tube

THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM 37

is one, if not the most striking, of the peculiarities which distinguish
the vertebrate ; a tubular central nervous system such as that of the
vertebrate is totally unlike any other nervous system, and the very
fact that the two nervous systems of the vertebrate and arthropod
are so similar in their nervous arrangements, makes it still more
extraordinary that the nervous system should be grouped round a
tube in the one case and not in the other.

Now, in the arthropod the oesophagus leads directly into the
stomach, which is situated in the head-region, and from this a straight
intestine passes directly along the length of the body to the anus,
where it terminates. The relations of mouth, oesophagus, alimentary
canal, and nervous system in these animals are represented in the
diagram (Fig. 3).

Any tube, therefore, such as that of the infundibulum, which
would represent the oesophagus of such an animal, must have opened
into the mouth on the ventral side, and into the stomach on the
dorsal side, and the lining epithelium of such an oesophagus must
have been continuous with that of the stomach, and so of the whole
intestinal tract.

Supposing, then, the animal is not turned over, but that the dorsal
side still remains dorsal and ventral ventral, then the original mouth-
opening of the oesophagus must be looked for on the ventral surface
of the vertebrate brain in the region of the pituitary body or hypo-
physis, and on the dorsal side the tube representing the oesophagus
must be continuous with a large cephalically dilated tube, which
ought to pass into a small canal, to run along the length of the body
and terminate in the anus.

This is exactly what is found in the vertebrate, for the infun-
dibular tube passes into the third ventricle of the brain, which forms,
with the other ventricles of the brain, the large dilated cephalic
portion of the so-called nerve tube, and at the junction of the medulla
oblongata and spinal cord, this dilated anterior part passes into the
small, straight, central canal of the spinal cord, which in the embryo
terminates in the anus by way of the neurenteric canal. If the
animal is regarded as not having been turned over, then the con-
clusion that the infundibulum was the original oesophagus leads
immediately to the further conclusion that the ventricles of the verte-
brate brain represent the original cephalic stomach, and the central
canal of the spinal cord the straight intestine of the arthropod ancestor.

38 THE ORIGIN OF VERTEBRATES

For the first time a logical, straightforward explanation is thus
given of the peculiarities of the tube of the central nervous system,
with its extraordinary termination in the anus in the embryo, its
smallness in the spinal cord, its largeness in the brain region, and its
offshoot to the ventral side of the brain as the infundibular channel.
It is so clear that, if the infundibular tube be looked on as the old
cesophagus, then its lining epithelium is the lining of that oesophagus ;
and the fact that this lining epithelium is continuous with that of
the third ventricle, and so with the lining of the whole nerve-tube,
must be taken into account and not entirely ignored as has hitherto
been the case. If, then, we look at the central nervous system of
the vertebrate in the light of the central nervous system of the
arthropod without turning the animal over, we are led immediately
to the conclusion that what has hitherto been called the vertebrate
nervous system is in reality composed of two parts, viz. a nervous
part comparable in all respects with that of the arthropod ancestor,
which has grown over and included into itself, to a greater or less
extent, a tubular part comparable in all respects with the alimentary
canal of the aforesaid ancestor. If this conclusion is correct, it is
entirely wrong to speak of the vertebrate central nervous system as
being tubular, for the tube does not belong to the nervous system,
but was originally a simple epithelial tube, such as characterizes the
cesophagus, cephalic stomach, and straight intestine of the arthropod.

Here, then, is the crux of the position — either the so-called
nervous tube of the vertebrate is composed of two separate factors,
consisting of a true non-tubular nervous system and a non-nervous
epithelial tube, these two elements having become closely connected
together; or it is composed of one factor, an epithelial tube which
constitutes the nervous system, its elements being all nervous
elements.

If this latter hypothesis be accepted, then it is necessary to
explain why parts of that tube, such as the roof of the fourth
ventricle, the choroid plexuses of the various ventricles, which are
parts of the original roof inserted into the ventricles, are not com-
posed of nervous material, but form simple single-layered epithelial
sheets, which by no possibility can be included among functional
nervous structures. The upholders of this hypothesis can only
explain the nature of these thin epithelial parts of the nervous tube
in one of two ways ; either the tube was originally formed of nervous

THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM 39

material throughout, and for some reason parts of it have lost their
nervous function and thinned down ; or else these thin epithelial
parts are on their way to become nervous material, are still in an
embryonic condition, and are of the nature of epiblast-epithelium,
from which the central nervous system originally arose.

The first explanation is said to be supported by embryology, for
at first the nerve-tube is formed in a uniform manner, and then
later, parts of the roof appear to thin out and so form the thin epi-
thelial parts. If this were the right explanation, then it ought to
be found that in the lowest vertebrates there is greater evidence of
a uniformly nervous tube than in the higher members of the group :
while conversely, if, on the contrary, as we descend the vertebrate
phylum, it is found that more and more of the tube presents the
appearance of a single layer of epithelium, and the nervous material
is limited more and more to certain parts of that tube, then the
evidence is strong that the tubular character of the central nervous
system is not due to an original nervous tube, but to a non-nervous
epithelial tube with which the original nervous system has become
closely connected.

The comparison of the brain region of the different groups of
vertebrates (Fig. 19) is most instructive, for it demonstrates in the
most conclusive manner how the roof of the nervous tube in that
region loses more and more its nervous character, and takes on the
appearance of a simple epithelial tube, as we descend lower and
lower ; until at last, in the brain of Ammoccetes, as represented in
the figures, the whole of the brain- roof, from the region of the
pineal eye to the commencement of the spinal cord, is composed of
fold upon fold of a thin epithelial membrane forming an epithelial
bag, which is constricted in only one place, where the fourth cranial
nerve crosses over it.

Further, the brain of Ammoccetes (Fig. 20) shows clearly not only
that it is composed of two parts, an epithelial tube and a nervous
system, but also that the nerve-masses are arranged in the same
relative position with respect to this tube as are the nerve-masses in
the invertebrate with respect to the cephalic stomach and cesophagus.
This evidence is so striking, so conclusive, that it is impossible to
resist the conclusion that the tube did not originate as part of the
central nervous system, but was originally independent of the central
nervous system, and has been invaded by it.

THE O RIG IX OF VERTEBRATES

MAMMALIA.

REPTILIA.

AMPHIBIA

TELEOSTEA

AMMOCCETES

Fig.19.— Comi'abisok of Vertebeate Bbalns.

CB., cerebellum ; FT., pituitary body ; PK., pineal body; C. STB., corpus striatum ;
G.H.B., right ganglion habenulse. I., olfactory; II., optic nerves.

CER

GHR

INF

CER

VII+VIII

Fig. 20. — Brain of
Ammoccetes.

A, dorsal view; B, late-
ral view; C, ventral
view.

Vll+Vlil

(B) xVff

-. v"\. •"*•

"Mi'

M •

■:■■&$

•v&.

.*>

- ' '*4j£i

.3)

•■-ii

C.E.H., cerebral hemi-
spheres ; G.H.R.,
right ganglion habe-
nulse ; PN., right
pineal eye ; CH„,
CH2, choroid plex-
uses ; I.— XII. cra-
nial nerves ; C.P.,
Conus post-commis-
suralis.

42 THE ORIGIN OF VERTEBRATES

The second explanation is hardly worth serious consideration, for
it supposes that the nervous system, for no possible reason, was laid
down in its most important parts — the brain-region — as an epithelial
tube with latent potential nervous functions ; that even up to the
highest vertebrate yet evolved these nervous functions are still in
abeyance over the whole of the choroid plexuses and the roof of the
fourth ventricle. Further, it supposes that this prophetic epithelial
tube originally developed into true nervous material only in certain
parts, and that these parts, curiously enough, formed a nervous
system absolutely comparable to that of the arthropod, while the
dormant prophetic epithelial part was formed so as just to mimic,
in relation to the nervous part, the alimentary canal of that same
arthropod.

The mere facts of the case are sufficient to show the glaring
absurdity of such an explanation. This is not the way Nature works ;
it is not consistent with natural selection to suppose that in a low
form nervous material can be laid down as non-nervous epithelial
material in order to provide in some future ages for the great increase
in the nervous system.

Every method of investigation points to the same conclusion,
whether the method is embryological, anatomical, or pathological.

First, take the embryological evidence. On the ground that the
individual development reproduces to a certain extent the phylo-
genetic development, the peculiarities of the formation of the central
nervous system in the vertebrate embryo ought to receive an appro-
priate explanation in any theory of phylogenetic development.
Hitherto such explanation has been totally lacking ; any suggestion
of the manner in which a tubular nervous system may have been
formed takes no account whatever of the differences between different
parts of the tube ; its dilated cephalic end with its infundibular
projection ventrally, its small straight spinal part, and its termination
in the anus. My theory, on the other hand, is in perfect harmony
with the embryological history, and explains it point by point.

From the very first origin of the central nervous system there
is evidence of two structures— the one nervous, and the other an
epithelial surface-layer which ultimately forms a tube ; this was
first described by Scott in Petromyzon, and later by Assheton in the
frog. In the latter case the external epithelial layer is pigmented,
while the underlying nervous layer contains no pigment ; a marked

THE EVIDENCE OE THE CENTRAL NERVOUS SYSTEM 43

and conspicuous demarcation exists, therefore, between the two layers
from the very beginning, and it is easy to trace the subsequent fate
of the two layers owing to this difference of pigmentation. The pig-
mented cells form the lining cells of the central canal, and becoming
elongated, stretch out between the cells of the nervous layer ; while
the latter, on their side, invade and press between the pigmented
cells. In this case, owing to the pigmentation of the epithelial layer,
embryology points out in the clearest possible manner how the
central nervous system of the vertebrate is composed of two struc-
tures— an epithelial non-nervous tube, on the outside of which the
central nervous system was originally grouped ; how, as develop-
ment proceeds, the elements of these two structures invade each
other, until at last they become so involved together as to give rise
to the conception that we are dealing with one single nerve tube.
It is impossible for embryology to give a clearer clue to the past
history than it does in this case, for it actually shows, step by step,
how the amalgamation between the central nervous system and the
old alimentary canal took place.

Further, consider the shape of the tube when it is first formed,
how extraordinary and significant that is. It consists of a simple
dilated anterior end leading into a straight tube, the lumen of which
is much larger than that of the ultimate spinal canal, and terminates
by way of the neurenteric canal in the anus.

Why should the tube take this peculiar shape at its first forma-
tion ? No explanation is given or suggested in any text-book of
embryology, and yet it is so natural, so simple : it is simply the shape
of the invertebrate alimentary canal with its cephalic stomach and
straight intestine ending in the anus. Again embryology indicates
most unmistakably the past history of the race. How are the
nervous elements grouped round this tube when it is first formed ?
Here embryology shows that a striking difference exists between the
part of the tube which forms the spinal cord and the dilated cephalic
part. Fig. 21, A (2), represents the relation between the nervous
masses and the epithelial tube in the first instance. At this stage
the nervous material in the spinal cord lies laterally and ventrally
to this tube, and at a very early stage the white anterior commissure
is formed, joining together these two lateral masses ; as yet there is
no sign of any posterior fissure, the tube with its open lumen extends
right to the dorsal surface.

THE ORIGIN OF VERTEBRATES

The interpretation of this stage is that in the invertebrate ancestor
the nerve-masses were situated laterally and ventrally to the
epithelial tube, and were connected together by commissures on the
ventral side of the tube (Fig. 21, A (1)) ; in other words, the chain of
ventral ganglia and their transverse commissures lying just ventrally
to the intestine, which are so characteristic of the arthropod nervous
system, is represented at this stage.

Subsequently, by the growth dorsalwards of nervous material to
form the posterior columns, the original epithelial tube is compressed
dorsally and laterally to such an extent that those parts lose all signs
of lumen, the one becoming the posterior fissure and the others the

3 J

Fig. 21. — A, Method of Formation of the Vertebrate Spinal Cord from the
Ventral Chain of Ganglia and the Intestine of an Arthropod, repre-
sented in 1 ; B, Method of Formation of the Vertebrate Medulla
Oblongata from the Infra-ossophageal Ganglia and the Cephalic
Stomach of an Arthropod.

substantia gelatinosa Rolandi on each side. The original tube is thus
reduced to a small canal formed by its ventral portion only (Fig. 21,
A (3)). In this way the spinal cord is formed, and the walls of the
original epithelial tube are finally visible only as the lining of the
central canal (Fig. 21, A (4)).

When we pass to the brain-region, to the anterior dilated
portion of the tube, embryology tells a different story. Here, as in
the spinal cord, the nervous masses are grouped at first laterally and
ventrally to the epithelial tube, as is seen in Fig. 21, B (2), but owing
to the large size of its lumen here, the nervous material is not
able to enclose it completely, as in the case of the spinal cord ;

THE FA7 IDE NCR OF THE CENTRAL NERVOUS SYSTEM 45

consequently there is no posterior fissure formed ; but, on the contrary,
the dorsal roof, not enclosed by the nerve-masses, remains epithelial,
and so forms the membranous roof of the fourth ventricle and of the
other ventricles of the brain (Fig. 21, B (3)). In the higher animals,
owing to the development of the cerebrum and cerebellum, this
membranous roof becomes pushed into the larger brain cavity, and
thus forms the choroid plexuses of the third and lateral ventricles.
In the lower vertebrates, as in Ammoccetes and the Dipnoi, it still
remains as a dorsal epithelial roof and forms a most striking
characteristic of such brains.

In this part of the nervous system, then, the nervous material is
all grouped in its original position on the ventral side of the tube ;
and yet it is the same nervous material as that of the spinal cord,
all the elements are there, giving origin here to the segmental cranial
nerves just as lower down they give rise to the segmental spinal
nerves, connecting together the separate segments each with the other
and all with the higher brain-centres — the supra-infundibular centres
— just as they do in the spinal region.

Why should there be this striking difference between the
formation of the infra-infundibular region of the brain and that of
the spinal cord ? Do the advocates of the origin of vertebrates from
Balanoglossus give the slightest reason for it ? They claim that their
view also provides a tubular nervous system for the vertebrate, but
give not the slightest sign or indication as to why the nervous
material should be grouped entirely on the ventral side of an
epithelial tube in the infra-infundibular region and yet surround
it in the spinal cord region. And the explanation is so natural,
so simple : embryology does its very best to tell us the past history
of the race, if only we look at it the right way.

The infra-infundibular nervous mass is naturally confined to the
ventral side of the epithelial tube, because it represents the infra-
cesophageal ganglia, situated as they are on the ventral side of the
cephalic stomach, and, owing to the size of the stomach, they could
not enclose it by dorsal growth, as they do in the case of the forma-
tion of the spinal cord (Fig. 21, B (1)). Still these nervous masses
have grown dorsalwards, have commenced to involve the walls of
the cephalic stomach even in the lowest vertebrate, as is seen in
Ammoccetes, in which animal a ventral portion of the epithelial
bag has been evidently compressed and its lumen finally obliterated

THE ORIGIN OF VERTEBRATES

by the growth of the nerve-masses on each side of it. Throughout
the whole vertebrate kingdom this obliterated portion still leaves

its mark as the raphe or seam,
which is so characteristic of
the infra-infundibular portion
of the brain.

Here, again, it is seen how
simple is the explanation of a
peculiarity which has always
puzzled anatomists — why
should there be this seam in
the infra-infundibular portion
of the brain and not in the
supra-infundibular or in the
spinal cord ? The correspond-
ing compression in the upper
brain-region forms the lateral
ventricles, as is seen in the
accompanying figure of the
brain of Ammoccetes (Fig. 22).
In yet another instance it is
seen how markedly the nervous
masses are arranged in the
same position with respect to
the central tube as are the
nerve ganglia with respect to

Fig. 22. — Horizontal Section through
the Brain of Ammoccetes.

Cr., membranous cranium ; I, olfactory
nerves; l.v., lateral ventricles; gl., glan-
dular tissue which fills up the cranial
cavity.

the intestinal tube in the case
of the invertebrate. Thus in birds a portion of the spinal cord
in the lumbo-sacral region presents a very different appearance

from the rest of the cord ; it is
known as the rhomboidal sinus,
and a section of the cord of an
adult pigeon across this region is
given in Fig. 23. As is seen, the
nervous portions are entirely con-
Pig. 23.— Section through Rhomboidal fined to two masses connected

together by the white anterior
commissures which are situated laterally and ventrally to a
median gelatinous mass ; the small central canal is visible and

THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM 47

the whole dorsal area of the cord is taken up by a peculiar non-
nervous wedge-shaped mass of tissue. At its first formation this
portion of the cord is formed exactly in the same manner as the rest
of the cord ; instead, however, of the nervous material invading the
dorsal part of the tube to form the posterior fissure, it has been from
some cause unable to do so, the walls of the original non-nervous
tube have become thickened dorsally, been transformed into this
peculiar tissue, and so caused the peculiar appearance of the cord
here. The nervous parts have not suffered in their development ;
the mechanism for walking in the bird is as well developed as in
any other animal ; their position only is different, for they still retain
the original ventro-lateral position, but the non-nervous tube, the
remains of the old intestine, has undergone a peculiar gelatinous
degeneration just where it has remained free from invasion by the
nervous tissue.

Throughout the whole of that part of the nervous system which
gives origin to the cranial and spinal segmental nerves, the evidence
is absolutely uniform that the nervous material was originally
arranged bilaterally and ventrally on each side of the central tube,
exactly in the same way as the nerve-masses of the infra-oesophageal
and ventral chain of ganglia are arranged with respect to the cephalic
stomach and straight intestine of the arthropod. But, in addition, we
find in the vertebrate nervous masses, the cerebral hemispheres, the
corpora quadrigemina and the cerebellum situated on the dorsal side
of the central tube in the brain-region ; this nervous material is,
however, of a different character to that which gives origin to the
spinal and cranial segmental nerves. How is the presence of these
dorsal masses to be explained on the supposition that the dilated
anterior part of the nerve-tube was originally the cephalic stomach
of the arthropod ancestor ? The cerebral hemispheres are simple
enough, for they represent the supra-cesophageal ganglia, which of
necessity, as they increased in size, would grow round the anterior
end of the cephalic stomach and become more and more dorsal in
position.

The difficulty lies rather in the position of the cerebellum and
corpora quadrigemina, and the solution is as simple as it is
conclusive.

Let us again turn to embryology and see what help it gives. In
all vertebrates the dilated anterior portion of the nerve-tube does not,

48 THE ORIGIN OF VERTEBRATES

as it grows, increase in size uniformly, but a constriction appears on
its dorsal surface at one particular place, so as to divide it into an
anterior and posterior vesicle ; then the latter becomes divided into
two portions by a second constriction. In this way three cerebral
vesicles are formed ; these three primary cerebral vesicles indicate
the region of the fore-brain, mid-brain, and hind-brain respectively.
Subsequently the first cerebral vesicle becomes divided into two to
form the prosencephalon and thalamencephalon, while the third
cerebral vesicle is also divided into two to form the region of the
cerebellum and medulla oblongata.

These constrictions are in the position of commissural bands of
nervous matter ; of these the limiting nervous strands between the
thalamencephalon and mesencephalon and between the mesencephalon
and the hind-brain are of primary importance. The first of these
commissural bands is in the position of the posterior commissure
connecting the two optic thalami. In close connection with this are
found, on the mid-dorsal region, the two pineal eyes with their optic
ganglia, the so-called ganglia habenulce. From these ganglia a
peculiar tract of fibre, known as Meynert's bundle, passes on each
side to the ventral infra-infundibular portion of the brain. In other
words, the first constriction of the dilated tube is due to the presence
and growth of nervous material in connection with the median pineal
eyes. Here in precisely the same spot, as will be fully explained
in the next chapter, there existed in the arthropod ancestor a pair
of median eyes situated dorsally to the cephalic stomach, the pre-
existence of which explains the reason for the first constriction.

The second primary constriction separating the mid-brain from
the hind-brain is still more interesting, for it is coincident with the
position of the trochlear or fourth cranial nerve. In all vertebrates
without exception this nerve takes an extraordinary course ; all other
nerves, whether cranial or spinal, pass ventralwards to reach their
destination. This nerve passes dorsalwards, crosses its fellow mid-
dorsally in the valve of Vieussens, where the roof of the brain is
thin, and then passes out to supply the superior oblique muscle of the
eye of the opposite side. The two nerves form an arch constricting
the dilated tube at this place. In the lowest vertebrate ( Ammoccetes)
the constriction formed by this nerve-pair is evident not only in the
embryonic condition as in other vertebrates, but during the whole
larval stage. As Fig. 20, A and B, shows, the whole of the dorsal

THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM 49

region of the brain up to the region of the pineal eye and ganglion
habenultc is one large membranous bag, except for the single con-
striction where the fourth nerve on each side crosses over. The
explanation of this peculiarity is given in Chapter VII., and follows
simply from the facts of the arrangement of that musculature in the
scorpion-group which gave rise to the eye-muscles of the vertebrate.

In Ammocoetes both cerebellum and posterior corpora quad-
rigemina can hardly be said to exist, but upon transformation a
growth of nervous material takes place in this region, and it is seen
that this commencing cerebellum and the corpora quadrigemina arise
from tissue that is present in Ammocoetes along the course of the
fourth nerve.

Here, then, again Embryology does its best to tell us how the
vertebrate arose. The formation of the two primary constrictions
in the dilated anterior vesicle whereby the brain is divided into
fore-brain, mid-brain, and hind-brain is simply the representation
ontogenetically of the two nerve-tracts which crossed over the
cephalic stomach in the prevertebrate stage, in consequence of
the mid-dorsal position of the pineal eyes and of the insertion of
the original superior oblique muscles.

The subsequent constriction by which the prosencephalon is
separated from the thalamencephalon is in the position of the
anterior commissure, that commissure which connects the two supra-
infundibular nerve-masses, and is one of the first-formed commis-
sures in every vertebrate. This naturally is simply the commissure
between the two supra-oesophageal ganglia; anterior to it, in the
middle line, equally naturally, the anterior end of the old stomach
wall still exists as the lamina terminalis.

The other division in the hind-brain region, which separates the
region of the cerebellum from the medulla oblongata, is due to the
growth of the cerebellum, and indicates its posterior limit. In such
an animal as the lamprey, where the cerebellum is only commencing,
this constriction does not occur in the embryo.

From such simple beginnings as are seen in Ammocoetes, the
higher forms of brain have been evolved, to culminate in that of man,
in which the massive cerebrum and cerebellum conceals all sio-n of
the dorsal membranous roof, those parts of the simple epithelial tul >e
which still remain being tucked away into the cavities to form the
various choroid plexuses.

THE ORIGIN OF VERTEBRATES

In the whole evolution from the brain of Ammocoetes to that of
man, the same process is plainly visible, viz. growth and extension
of nervous material over the epithelial tube; extension dorsally and
posteriorly of the supra-infundibular nervous masses (as seen in
Fig. 19), combined with a dorsal growth of parts of the infra-
infundibular nervous masses to form the cerebellum and posterior
corpora quadrigemina.

Espceially instructive is the formation of the cerebellum. It
consists at first of a small mass of nervous tissue accompanying the

fourth nerve, then by the growth of that mass
surrounding and constricting a fold of the
membranous roof, the worm of the cerebellum
is formed, as in the dog-fish. This very con-
striction causes the membrane to be thrown
into a lateral fold on each side, as seen in
Fig. 24, and in the dog-fish the nervous material
on each side, known as the fimbriae, is already
commencing to grow from the ventral mass of
the medulla oblongata to surround these lateral
membranous folds. These fimbriae develop more
and more in higher forms, and thus form the
cerebellar hemispheres.

Not only does comparative anatomy confirm
the teachings of embryology, but also pathology
gives its quota in the same direction.

One of the striking facts about malforma-
tions and disease of the central nervous system
is the frequency of cystic formations ; spina
bifida is a well-known instance. These cysts are merely epithelial
non-nervous cysts formed from the epithelium of the central canal,
dilficult to understand if the whole nerve tube is one and entirely
nervous, either actually or potentially, but natural and easy if we
are really dealing with a simple epithelial tube on the outside of
which the nervous material was originally grouped. The cystic
formation belongs naturally enough to this tube, not to the nervous
system.

Again, where animals such as lizards have grown a new tail,
owing to the breaking off of the original one, it is found that the
central canal extends into this new tail for some distance, but not

Fig. 24. — Cebebel-
lum of Dog-fish.

v, worm of cerebel-
lum; IV., membra-
nous roof of fourth
ventricle continuous
with the membra-
nous folds on each
side. Through these
the fimbrise (fb.) can
be dimly seen.

THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM 51

the nervous material surrounding it ; all the nerves supplying the
new tail arise from the uninjured spinal cord above, the central
canal with its lining layer of epithelial cells alone grows into the
new-formed appendage.

To all intents and purposes the same thing is seen in the termi-
nation of the spinal cord in a bird-embryo; more and more, as the
end of the tail is approached, does the nervous matter of the spinal
cord grow less and less, until at last a naked central canal with
its lining epithelium is alone left to represent the so-called nerve-
tube.

All these different methods of investigation lead irresistibly to
the one conclusion that the tubular nature of the central nervous
system has been caused by the central nervous system enclosing to a
greater or less extent a pre-existing, non-nervous, epithelial tube.

This must always be borne strictly in mind. The problem, there-
fore, which presents itself is the comparison of these two factors
separately, in order to find out the relationship of the vertebrate to
the invertebrate. The nervous system without the tube must be
compared to other nervous systems, and the tube must be considered
apart from the nervous system.

The Principle of Concentration and Cefhalizatiox.

The central nervous system of the vertebrate resembles that of
all the Appendiculata in the fact that it is composed of segments
joined together which give origin to segmental nerves. There is,
however, a great difference between the two systems : the division
into separate segments is not obvious to the eye in the vertebrate
nervous system, while in the invertebrate we can see that it is
composed of a series of separate pairs of ganglia joined together
longitudinally by nervous strands known as connectives and trans-
versely by the nerve-commissures. Such a simple segmented system
is found in the segmented worms, and in the lower arthropods, such
as Branchipus, no great advance has been made on that of the annelid.
In the higher forms, however, a greater and greater tendency to fusion
of separate ganglia exists, especially in the head-region, so that the
infra- (esophageal ganglia, which, in the lower forms are as separate
as those of the ventral chain, in the higher forms are fused together
to form a single nervous mass.

52 THE ORIGIN OF VERTEBRATES

This is the great characteristic of the advancement of the central
nervous system among the Invertebrata, its concentration in the
region of the head. It may be called the principle of cephalization,
and is characteristic not only of higher organization in a group, but
also of the adult as distinguished from the larval form. Thus in the
imago greater concentration is found than in the caterpillar.

The segmented annelid type of nervous system consists of a
supra-oesophageal ganglion, composed of the fused ganglia belonging
to the pre-oral segments, and an infra-cesophageal chain of separate
ganglia. With the concentration and modification around the
mouth of the most anterior locomotor appendages to form organs
for prehension and mastication of food, a corresponding concentra-
tion and fusion of the ganglia belonging to these segments takes
place, so that finally, in the higher annelids, and in most of the great
arthropod group, a fusion of a number of the most anterior ganglia
has taken place to form the infra-cesophageal ganglion-mass.

The infra-cesophageal ganglia which are the first to fuse are
those which supply the most anterior portion of the animal with
nerves, and include always those anterior appendages which are
modified for mastication purposes. To this part the name pivsoma
has been given ; in many cases it forms a well-defined, distinct
portion of the animal.

Succeeding this prosoma or masticatory region, there occurs in
all gill-bearing arthropods a respiratory region, in many cases more
or less distinctly defined, which has received the name of mcsosoma.
The rest of the body is called the metasoma.

In accordance with this nomenclature the central nervous system
of many of the Arthropoda may be divided as follows : —

1. Pre-oral, or supra-oesophageal ganglia.

2. Infra-oral, or infra-cesophageal ganglia and ventral chain,
which consist of three groups : prosomatic, mesosomatic, and meta-
somatic ganglia.

The infra-cesophageal ganglion- mass, then, in most of the Arthro-
poda may be spoken of as formed by the fusion of the prosomatic or
mouth-ganglia, the mesosomatic and metasomatic remaining separate
and distinct. The number of ganglia which have fused may be
observed by examination of the embryo, in which it is easy to see
indications of the individual ganglia or ncuromercs, although all
such indication has disappeared in the adult ; thus the infra-ceso-

THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM 53

phageal ganglia of the cray-fish have been shown to be constituted
of six prosomatic ganglia.

In Fig. 25 I give figures of the central nervous system (with the
exception of the abdominal or metasomatic ganglia) of Branchipus,
Astacus, Limulus, Scorpio, Androctonus, Thelyphonus, and Ammo-
ccetes. In all the figures the supra-cesophageal ganglia are lined
horizontally, and their nerves shown, viz. optic (lateral eyes (II) and
median eyes (II')), olfactory (I) (first antenna?, camerostome, nose);
then come the prosomatic ganglia (dotted), with their nerves (A)
supplying the mouth parts, and the second antenna? or chelicera? ;
then the mesosomatic (lined horizontally), with their nerves (B)
supplying respiratory appendages. These figures show that the con-
centrated brain mass around the oesophagus of an arthropod which
has arrived at the stage of Astacus, is represented by the supra-
ossophageal ganglia and the fused prosomatic ganglia.

The next stage in the evolution of the brain is seen in the
gradual in lusion of the mesosomatic ganglia, one after the other,
into the infra-cesophageal mass of the already fused prosomatic
ganglia. "With this fusion is associated the loss of locomotion in
these mesosomatic appendages, and their entire subservience to the
function of respiration. Dana urges that cephalization is a conse-
quence of functional alteration in the appendages, from organs of
locomotion to those of mastication and respiration. Whether this be
true or not, it is certainly a fact that in Limulus, the ganglion
supplying the first mesosomatic appendage has fused with the
prosomatic, infra-cesophageal mass. It is also a fact that the proso-
matic appendages are the organs of mastication, their basal parts
being arranged round the mouth so as to act as foot-jaws, while the
mesosomatic appendages, though still free to move, have been
reduced to such an extent as to consist mainly of their basal parts,
which are all respiratory in function, except in the case of the first
pair, where they carry the terminal ducts of the genital organs. In
the next stage, that, of the scorpion, in which the mesosomatic
appendages have lost all power of free locomotion, and have become
internal branchiae, another mesosomatic ganglion has fused with the
brain mass, while in Androctonus two of the branchial mesosomatic
ganglia have fused ; and finally, in Thelyphonus and Phrynus, all
the mesosomatic ganglia have coalesced with the fused prosomatic
ganglia, while the metasomatic ganglia have themselves fused

THE O RIG IS OF VERTEBRATES

ANDROCTONUS

AMMOCCETES

Fig. 25. — Comparison of Invertebrate Brains from Branchipus to

Ammoccetes.

THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM 55

together in the caudal region to form what is known as the caudal
brain.

The brain in these animals may be spoken of as composed of
three parts — (1) the fused supra-cesophageal ganglia, (2) the fused
prosomatic ganglia, and (3) the fused mesosomatic ganglia. Such a
brain is strictly homologous with the vertebrate brain, which also is
built up of three parts — (1) the part in front of the notochord, the
prechordal or supra-infundibular brain, which consists of the cerebral
hemispheres, together with the basal and optic ganglia and corre-
sponds, therefore, to the supra-cesophageal mass, with its olfactory
and optic divisions lying in front of the oesophagus ; (2 and 3) the
epichordal brain, composed of (2) a trigeminal and (3) a vagus divi-
sion, of which the first corresponds strictly to the fused prosomatic
ganglia, and the second to the fused mesosomatic ganglia. Further,
just as in the embryo of an arthropod it is possible, with more or
less accuracy, to see the number of neuromeres or original ganglia
which have fused to form the supra- and infra- oesophageal portions
of its brain, so also in the embryo of a vertebrate we are able at
an early stage to gain an indication, more or less accurate, of the
number of neuromeres which have built up the vertebrate brain.
The further consideration of these neuromeres, and the evidence they
afford as to the number of the prosomatic and mesosomatic ganglia
which have formed the epichordal part of the vertebrate brain, must
be left to the chapter on the segmentation of the cranial nerves.

The further continuation of this process of concentration of
separate segments, together with the fusion of the nervous system
with the tube of the alimentary canal, leads in the simplest manner
to the formation of the spinal cord of the vertebrate from the meta-
somatic ganglia of the ventral chain of the arthropod.

The Antagonism between Cephalization and Alimentation.

This concentration of the nervous system in the head- region,
together with an actual increase in the bulk of the cephalic nervous
masses, constitutes the great principle upon which the law of upward
progress or evolution in the animal kingdom is based, and it illus-
trates in a striking manner the blind way in which natural selection
works; for, as already explained, the central nervous system arose as
a ring round the mouth, in consequence of which, with the progressive

THE ORIGIN OF VERTEBRATES

evolution of the animal kingdom, the oesophagus necessarily pierced
the central nervous system at the cephalic end. At the same time,
the very fact that the evolution was progressive necessitated the
concentration and increase of the nervous masses in this very same
oesophageal region.

Progress on these lines must result in a crisis, owing to the
inevitable squeezing out of the food-channel by the increasing nerve-
mass ; and, indeed, the fact that such a crisis had in all probability
arisen at the time when vertebrates first appeared is apparent when
we examine the conditions at the present time.

Those invertebrates whose central nervous system is most con-
centrated at the cephalic end belong to the arachnid group, among
which are included the various living scorpion-like animals, such as
Thelyphonus, Androctonus, etc.

As already mentioned, the giants of the Palaeostracan age were

Pterygotus, Slimonia, etc., all animals of the scorpion-type — in fact,

A sea - scorpions. Now, all these

,S "•'.. animals, spiders and scorpions,

without exception, are blood -
suckers, and in all of them the
concentrated cephalic mass of ner-
vous material surrounds an oeso-
phagus the calibre of which is so
small that nothing but a fluid
pabulum can be taken into the
alimentary canal ; and even for
that purpose a special suctorial
apparatus has in some species
been formed on the gastric side
of the oesophagus for the purpose
of drawing blood through this

Fig. 26. — Transverse Section
through the brain of a young
Thelyphonus.

exceedingly narrow tube.

increasing

In Fig. 25 this
antagonism between brain-power
and alimentation, as we pass from
such a form as Branchipus to the
scorpion, is illustrated, and in Fig. 26 the relative sizes of the
oesophagus and the brain-mass surrounding it is shown. The section
shows that the food channel is surrounded by the white and grey

-4, supra-oesophageal ganglia; B, infra
oesophageal ganglia; Al, cesopkagus.

THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM 57

matter of the brain as completely as the central canal of the spinal
cord of the vertebrate is surrounded by the white and grey nervous
material.

Truly, at the time when vertebrates first appeared, the direction
and progress of variation in the Arthropoda was leading, owing to
the manner in which the brain was pierced by the oesophagus, to a
terrible dilemma — either the capacity for taking in food without
sufficient intelligence to capture it, or intelligence sufficient to capture
food and no power to consume it.

Something had to be done — some way had to be found out of this
difficulty. The atrophy of the brain meant degeneration and the
reduction to a lower stage of organization, as is seen in the Tunicata.
The further development of the brain necessitated the establish-
ment of a new method of alimentation and the closure of the old
oesophagus, its vestiges still remaining as the infundibular canal of
the vertebrate, meant the enormous upward stride of the formation
of the vertebrate.

At first sight it might appear too great an assumption even to
imagine the possibility of the formation of a new gut in an animal so
highly organized as an arthropod, but a little consideration will, I
think, show that such is not the case.

In the higher animals we are accustomed to speak of certain
organs as vital and necessary for the further existence of the animal ;
these are essentially the central nervous system, the respiratory
system, the circulatory system, and the digestive system. Of these
four vital systems the first cannot be touched without the chance
of degeneration ; but that is not the case with the second. The
passage from the fish to the amphibian, from the water-breathing
to the air-breathing animal, has actually taken place, and was effected
by the modification of the swim-bladder to form new respiratory
organs — the lungs ; the old respiratory organs — the gills — becoming
functionless, but still persisting in the embryo as vestiges. The
necessity arose in consequence of the passage of the animal from
water to land, and with this necessity nature found a means of over-
coming the difficulty ; air-breathing vertebrates arose, and from the
very fact of their being able to extend over the land-surfaces,
increased in numbers and developed in complexity in the manner
already sketched out.

For a respiratory system all that is required is an arrangement

58 THE ORIGIN OF VERTEBRATES

by means of which blood should be brought to the surface, so as to
interchange its gases with those of the external medium ; and it is
significant to find that of all vertebrates the Amphibia alone are
capable of an effective respiration by means of the skin.

As to the circulatory system, it is exceedingly easily modified.
An animal such as Amphioxus has no heart ; in some the heart is
systemic, in others branchial ; in some there are more than one heart ;
in others there are contractile veins in addition to a heart. There
is no difficulty here in altering and modifying the system according
to the needs of the individual.

For a digestive system all that is required is an arrangement for
the digestion and absorption of food, a mechanism which can arise
easily if some of the cells of the skin possess digestive power. Now
Miss Alcock has shown that some of the surface-cells of crustaceans
secrete a fluid which possesses digestive powers, and she has also
shown that certain of the cells in the skin of Ammocoetes possess
digestive power.

The difficulty, then, of forming a new digestive system in the
passage from the arthropod to the vertebrate is very much the same
as the difficulty in forming a new respiratory system in the passage
from the water-breathing fish to the air-breathing amphibian — a
change which does not strike us as inconceivable, because we know it
has taken place.

The whole argument so far leads to the conclusion that vertebrates
arose from ancient forms of arthropods by the formation of a new
alimentary canal, and the enclosure of the old canal by the growing
central nervous system. If this conclusion is true, then it follows
that we possess a well-defined starting-point from which to compare
the separate organs of the arthropod with those of the vertebrate,
and if, in consequence of such working hypothesis, each organ of the
arthropod is found in the vertebrate in a corresponding position and
of similar structure, then the truth of the starting-point is proved as
fully as can possibly be expected by deductive methods. It is, in
fact, this method of comparative anatomy which has proved the
descent of man from the ape, the frog from the fish, etc.

Let us, then, compare all the organs of such a low vertebrate as
Ammocoetes with those of an arthropod of the ancient type.

THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM 59

Life History of the Lamprey — not a Degenerate Animal.

The striking peculiarity of the lamprey is its life-history. It
lives in fresh water, spending a large portion of its life in the mud
during the period of its larval existence : then comes a somewhat
sudden transformation-stage, characterized, as in the lepidopterous
larva, by a process of histolysis, by which many of the larval tissues
are destroyed and new ones formed, with the result that the larval
lamprey, or Ammoccetes, is transformed into the adult lamprey, or
Petromyzon. This transformation takes place in August, at all
events in the neighbourhood of Cambridge, and later in the year the
transformed lamprey migrates to the sea, grows in size and maturity,
and returns to the river the following spring up to its spawning beds,
where it spawns and forthwith dies. How long it lives in the Ammo-
coetes stage is unknown ; I myself have kept some without transfor-
mation for four years, and probably they live in the rivers longer
than that before they change from their larval state. It is absolutely
certain that very much the longest part of the animal's life is spent
in the larval stage, and that with the maturity of the sexual organs
and the production of the fertilized ova the life of the individual ends.

Now, the striking point of this transformation is that it produces
an animal more nearly comparable with higher vertebrates than is
the larval form ; in other words, the transformation from larva to
adult is in the direction of upward progress, not of degeneration.
It is, therefore, inaccurate to speak of the adult lamprey as
degenerate from a higher race of fishes represented by its larval form
— Ammoccetes. Its transformation does not resemble that of the
tunicates, but rather that of the frog, so that, just as in the case of
the tadpole, the peculiarities of its larval form may be expected to
afford valuable indications of its immediate ancestry. The very
peculiarities to which attention must especially be paid are those
discarded at transformation, and, as will be seen, these are essentially
characteristic of the invertebrate and are not found in the higher
vertebrates. In fact, the transformation of the lamprey from the
Ammoccetes to the Petromyzon stage may be described as the casting
off of many of its ancestral invertebrate characters and the putting
on of the characteristics of the vertebrate type. It is this double
individuality of the lamprey, together with its long-continued
existence in the larval form, which makes Ammoccetes more

60 THE ORIGIN OF VERTEBRATES

valuable than any other living vertebrate for the study of the stock
from which vertebrates sprang.

Many authorities hold the view that the lamprey, like Amphioxus,
must he looked upon as degenerate, and therefore as no more suitable
for the investigation of the problem of vertebrate ancestry than is
Amphioxus itself. This charge of degeneracy is based on the state-
ment that the lamprey is a parasite, and that the eyes in Ammoccetes
are under the skin. The whole supposition of the degeneracy of the
Cyclostomata arose because of the prevailing belief of the time that
the earliest fishes were elasmobranchs, and therefore gnathosto-
matous. From such gnathostomatous fishes the cyclostomes were
supposed to have descended, having lost their jaws and become
suctorial in habit in consequence of their parasitism.

The charge of parasitism is brought against the lamprey because
it is said to suck on to fishes and so obtain nutriment. It is, how-
ever, undoubtedly a free-swimming fish ; and when we see it coming
up the rivers in thousands to reach the spawning-beds, and sucking
on to the stones on the way in order to anchor itself against the
current, or holding on tightly during the actual process of spawning,
it does not seem justifiable to base a charge of degeneration upon a
parasitic habit, when such so-called habit simply consists in holding
on to its prey until its desires are satisfied. If, of course, its suctorial
mouth had arisen from an ancestral gnathostomatous mouth, then
the argument would have more force.

Dohrn, however, gives absolutely no evidence of a former
gnathostomotous condition either in Petromyzon or, in its larval
state, Ammoccetes. He simply assumes that the Cyclostomata are
degenerated fishes and then proceeds to point out the rudiments of
skeleton, etc., which they still possess. Every point that Dohrn
makes can be turned round ; and, with more probability, it can be
argued that the various structures are the commencement of the
skeletal and other structures in the higher fishes, and not their
degenerated remnants. Compare the life-history of the lamprey
and of the tunicate. In the latter case we look upon the animal as
a degenerate vertebrate, because the larval stage alone shows verte-
brate characteristics ; when transformation has taken place, and the
adult form is reached, the vertebrate characteristics have vanished,
and the animal, instead of reaching a higher grade, has sunk lower
in the scale, the central nervous system especially having lost all

THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM 6 1

resemblance to that of the vertebrate. In the former case a trans-
formation also takes place, a marvellous transformation, characterized
by two most striking facts. On the one hand, the resulting animal
is more like a higher vertebrate, for, by the formation of new
cartilages, its cranial skeleton is now comparable with that of the
higher forms, and the beginnings of the spinal vertebrae appear ; by
the increased formation of nervous material, its brain increases in
size and complexity, so as to compare more closely with higher
vertebrate brains ; its eyes become functional, and its branchiae are
so modified, simultaneously with the formation of the new alimentary
canal in the cranial region, that they now surround branchial pouches
which are directly comparable to those of higher vertebrates. On
the other hand, the transformation process is equally characterized
by the throwing off of tissues and organs, one and all of which are
comparable in structure and function with corresponding structures in
the Arthropoda — the thyroid of the Ammoccetes, the tentacles, the
muco-cartilage, the tubular muscles, all these structures, so striking
in the Ammoccetes stage, are got rid of at transformation. Here is
the true clue. Here, in the throwing off of invertebrate characters,
and the taking on of a higher vertebrate form, especially a higher
brain, not a lower one, Petromyzon proclaims as clearly as is possible
that it is not a degenerate elasmobranch, but that it has arisen from
Ammocoetes-like ancestors, even though Myxine, Amphioxus, and
the tunicates be all stages on the downward grade from those same
Ammoccetes-like ancestors.

As to the eyes, they are functional in the adult form and as service-
able as in any fish. There is no sign of degeneracy; it is only possible
to speak of a retarded development which lasts through the larval stage.

Comparison of Brain of Ammocqites with that of an

Arthropod,

Seeing that the steady progress of the development of the central
nervous system is the most important factor in the evolution of
animals, it follows that of all organs of the body, the central nervous
system must be most easily comparable with that of the supposed
ancestor. I will, therefore, start by comparing the brain of
Ammocoetes with that of arthropods, especially of Limulus and of
the scorpion-group.

THE ORIGIN OF VERTEBRATES

The supra-infundibular portion of the brain in vertebrates
corresponds clearly to the supra-cesophageal portion of the inverte-
brate brain in so far that in both cases here is the seat of the will.
Voluntary action is as impossible to the arthropod deprived of its
supra-cesophageal ganglia as to the vertebrate deprived of its cere-
brum. It corresponds, also, in that from it arise the nerves of sight
and smell and no other nerves ; this is also the case with the supra-
cesophageal ganglia, for from a portion of these ganglia arise the nerves
to the eyes and the nerves to the first antennte, of which the latter
are olfactory in function. Thus, in the accompanying figure, taken
from Bellonci, it is seen that the supra-cesophageal ganglia consist

Sup. Segment Ant I

Ant II

Inf. Segment

Fig. 27. — The Brain op Sphceroma serratum. (After Bellonci.)

Ant. I. and Ant. II., nerves to 1st and 2nd antenna?, f.br.r., terminal fibre layer of
retina; Op. g. I., first optic ganglion; Op. g. II., second optic ganglion; O.n.,
optic nerve-fibres forming an optic cbiasma.

of a superior segment corresponding to the cerebrum, a middle
segment from which arise the nerves to the lateral eyes and to the
olfactory antennas, corresponding to the basal ganglia of the brain
and the optic lobes, and, according to Bellonci, of an inferior segment
from which arise the nerves to the second pair of antennae. This
last segment is not supra-cesophageal in position, but is situated on
the oesophageal commissures. It has been shown by Lankester and
Brauer in Limulus and the scorpion to be in reality the first ganglion
of the infra-cesophageal series, and not to belong to the supra-
cesophageal group.

Further, in Limulus, in the scorpion-group, and in all the extinct

THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM 6

Eurypteridce— in fact, in the Palaaostraca generally — there are two
median eyes in addition to the lateral eyes, which were innervated
from these ganglia.

In Ammoccetes, then, if the supra-infundibular portion of the
brain really corresponds to the supra-cesophageal of the paleeostracan
group, we ought to find, as indeed is the case, an optic apparatus
consisting of two lateral eyes and two median eyes, innervated from
the supra-infundibular brain-mass, and an olfactory apparatus built
up on the same lines as in the scorpion-group, also innervated from
this region. If, in addition, it be found that those two median eyes
are degenerate eyes of the same type as the median eyes of Limulus
and the scorpion-group, then the evidence is so strong as to amount
to a proof of the correctness of the theory. This evidence is precisely
what has been obtained in recent years, for the vertebrate did possess
two median eyes in addition to the two lateral ones, and these two
median eyes are degenerate eyes of the type found in the median
eyes of arthropods and are not of the vertebrate type. Moreover, as
ought also to be the case, they are most evident, and one of the
pair is most nearly functional in the lowest perfect vertebrate,
Anmiocoetes.

Of all the discoveries made in recent years, the discovery that
the pineal gland of the vertebrate brain was originally a pair of
median eyes is by far the most important clue to the ancestry of
the vertebrate, for not only do they correspond exactly in position
with the median eyes of the invertebrates, but, being already
degenerate and functionless in the lowest vertebrate, they must have
been functional in a pre-vertebrate stage, thus giving the most direct
clue possible to the nature of the pre-vertebrate stage. It is
especially significant that in Limulus they are already partially
degenerated. What, then, ought to.be the structure and relation to
the brain of the median and lateral eyes of the vertebrate if they
originated from the corresponding organs of some one or other member
of the paheostracan group ?

This question will form the subject of the next chapter.

Summary.

The object of this book is to attempt to find out from what group of inverte-
brates the vertebrate arose ; no attempt is made to speculate upon the causes of
variation by means of which evolution takes place.

64 THE O RIG IX OF VERTEBRATES

A review of the animal kingdom as a whole leads to the conclusion that the
upward development of animals from an original coelenterate stock, in which
the central nervous system consists of a ring of nervous material surrounding
the mouth, has led. in consequence of the elaboration of the central nervous
system, to a general plan among the higher groups of invertebrates in the topo-
graphical arrangement of the important organs. The mouth is situated ventrally.
and leads by means of the oesophagus into an alimentary canal which is situated
dorsally to the central nervous system. Thus the oesophagus pierces the central
nervous system and divides it into two parts, the supra-oesophageal ganglia
and the infra-cesophageal gangdia. This is an 'almost universal plan among
invertebrates, but apparently does not hold for vertebrates, for in them the
central nervous system is always situated dorsally and the alimentary canal
ventrally, and there is no piercing of the central nervous system by an oesophagus.

Yet a remarkable resemblance exists between the central nervous system of
the vertebrate and that of the higher invertebrates, of so striking- a character as
to compel one school of anatomists to attempt the derivation of vertebrates
from annelids. Now, the central nervous system of vertebrates forms a hollow
tube, and a diverticulum of this hollow tube, known as the infundibulum, passes
to the ventral surface of the brain in the very position where the oesophagus
would have been if that brain had belonged to an annelid or an arthropod.
This school of anatomists therefore concluded that this infundibular tube
rejn'esented the original invertebrate oesophagus which had become closed and
no longer opened into the alimentary canal owing to the formation of a new
niouth in the vertebrate. As, however, the alimentary canal of the vertebrate
is ventral to the central nervous system, and not dorsal, as in the invertebrate,
it follows that the remains of the original invertebrate mouth into which the
oesophagus (in the vertebrate the infundibular tube) must have opened must be
searched for on the dorsal side of the vertebrate ; and so the theory was put
forward that the vertebrate had arisen from the annelid by the reversal of
surfaces, the back of the one animal becoming the front of the other.

The difficulties in the way of accepting such reversal of surfaces have proved
insuperable, and another school has arisen which suggests that evolution has
throughout proceeded on two lines, the one forming- g-roups of animals in which
the central nervous system is pierced by the food-channel and the gut therefore
lies dorsally to it, the other in which the central nervous system always lies
dorsally to the alimentary canal and is not pierced by it. In both cases the
highest products of the evolution have become markedly segmented animals, in
the former, annelids and arthropods ; in the latter, vertebrates. The only
evidence on which such theory is based is the existence of low forms of animals,
known as the Enteropneusta, the best known example of which is called
BalauiHjlossiis ; they are looked upon as aberrant annelid forms by many
observers.

This theoiy does not attempt to explain the peculiarities of the tube of the
vertebrate central nervous system, or to account for the extraordinary resemblance
between the structure and arrangement of the central nervoiis systems of
vertebrates and of the highest invertebrate group.

Neither of these theories is satisfactory or has secured universal acceptance.
The problem must be considered entirely anew. What are the g-uiding principles
in this investigation ?

THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM 65

The evolution of animal life on this earth can clearly, on the whole, he
described as a process of upward progress culminating in the highest form —
man ; but it must always be remembered that whole groups of animals such as the
Tunicata have been able to survive owing to a reverse process of degeneration.

If there is one organ more than another which increases in complexity as
evolution proceeds, which is the most essential organ for upward progress, surely
it is the central nervous system, especially that portion of it called the brain.
This consideration points directly to the origin of vertebrates from the most
highly organized invertebrate group — the Arthropoda — for among all the groups
of animals living on the earth in the present day they alone possess a central
nervous system closely comparable with that of vertebrates. Not only has an
upward progress taken place in animals as a whole, but also in the tissues them-
selves a similar evolution is apparent, and the evidence shows that the vertebrate
tissues resemble more closely those of the arthropod than of any other inverte-
brate group.

The evi deuce of geology points to the same conclusion, for the evidence of
the rocks shows that before the highest mammal — man— appeared, the dominant
race was the mammalian quadruped, from whom the highest mammal of all —
man — sprung ; then comes, in Mesozoic times, the age of reptiles which were
dominant when the mammal arose from them. Preceding this era we find in
Carboniferous times that the amphibian was dominant, and from them the next
higher group — the reptiles — arose. Below the Carboniferous come the Devonian
strata with their evidence of the dominance of the fish, from whom the
amphibian was directly evolved. The evidence is so clear that each succeeding
higher form of vertebrate arose from the highest stage reached at the time,
as to compel one to the conclusion that the fishes arose from the race which
was dominant at the time when the fishes first appeared. This brings us to the
Silurian age, in which the evidence of the rocks points unmistakably to the sea-
scorpions, king-crabs, and trilobites as being the dominant race. It was preceded
by the great trilobite age, and the whole period, from the first appearance of the
trilobite to the time of dwindling away of the sea-scorpions, may be designated
the Pakeostracan age, using the term Palaeostraca to include both trilobites and
the higher scorpion and king-crab forms evolved from them. The evidence of
geology then points directly and strongly to the origin of vertebrates from the
Palaeostraca — arthropod forms which were not crustacean and not arachnid,
but gave origin both to the modern-day crustaceans and arachnids. The
history of the rocks further shows that these ancient fishes, when they first
appeared, resembled in a remarkable manner members of the palaeostracan group,
so that again and again paleontologists have found great difficulty in determin-
ing whether a fossil is a fish or an arthropod. Fortunately, there is still alive
on the earth one member of this remarkable group— the Limulus, or King-
Crab. On the vertebrate side the lowest non-degenerate vertebrate is the
lamprey, or Petromyzon, which spends a large portion of its existence in a
larval stage, known as the Ammoccetes stage of the lamprey, because it was
formerly considered to be a separate species and received the name of
Ammoccetes. The larval stages of any animal are most valuable for the study
of ancestral problems, so that it is most fortunate for the solution of the ancestry
of vertebrates that Limulus on the one side and Ammoccetes on the other are

66 THE ORIGIN OF VERTEBRATES

available for thorough investigation and comparison. There are no trilobites
still alive, but in Branchipus and Apus we possess the nearest approach to the
trilobite organization among living crustaceans.

So strongly do all these different lines of argument point to the origin of
vertebrates from arthropods as to make it imperative to reconsider the position
of that school of anatomists who derived vertebrates from annelids by reversing
the back and front of the animal. Let us not turn the animal over, but
re-consider the position, the infundibular tube of the vertebrate still representing
the oesophagus of the invertebrate, the cerebral hemispheres and basal ganglia
the supra-oesophageal ganglia, the crura cerebri the oesophageal commissures,
and the infra-infundibular part of the brain the infra-oesophageal ganglia. It
is immediately apparent that just as the invertebrate oesophagus leads into the
large cephalic stomach, so the infundibular tube leads into the large cavity of
the brain known as the third ventricle, which, together with the other ventricles,
forms in the embryo a large anterior dilated part of the neural tube. In the
arthropod this cephalic stomach leads into the straight narrow intestine ; in the
vertebrate the fourth ventricle leads into the straight narrow canal of the spinal
cord. In the arthropod the intestine terminates in the anus ; in the vertebrate
embryo the canal of the spinal cord terminates in the anus by way of the
neurenteric canal. Keep the animal unreversed, and immediately the whole
mystery of the tubular nature of the central nervous system is revealed, for it
is seen that the nervous matter, which corresponds bit by bit with that of the
arthropod, has surrounded to a greater or less extent and amalgamated with
the tube of the arthropod alimentary canal, and thus formed the so-called
central nervous system of the vertebrate.

The manner in which the nervous material has invaded the walls of the tube
is clearly shown both by the study of the comparative anatomy Of the central
nervous system in the vertebrate and also by its development in the embryo.

This theory implies that the vertebrate alimentary canal is a new formation
necessitated by the urgency of the case, and, indeed, there was cause for urgency,
for the general plan of the evolution of the invertebrate from the ccelenterate
involved the piercing of the anterior portion of the central nervous system by the
oesophagus, while, at the same time, upward progress meant brain-development ;
brain-development meant concentration of nervous matter at the anterior end
of the animal, with the result that in the highest scorpion and spider-like
animals the brain-mass has so grown round and compressed the food-tube that
nothing but fluid pabulum can pass through into the stomach ; the whole group
have become blood-suckers. These kinds of animals — the sea-scorpions — were
the dominant race when the vertebrates first appeared : here in the natural com-
petition among members of the dominant race the difficulty must have become
acute. Further upward evolution demanded a larger and larger brain with the
ensuing consequence of a greater and greater difficulty of food-supply. Nature's
mistake was rectified and further evolution secured, not by degeneration in the
brain-region, for that means degradation not upward progress, but by the
formation of a new food-channel, in consequence of which the brain was free
to develop to its fullest extent. Thus the great and mighty kingdom of the
Vertebrata was evolved with its culminating organism — man — whose massive
brain with all its possibilities could never have been evolved if he had still been

THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM 6 J

compelled to pass the whole of his food through the narrow oesophageal tube,
still existent in him as the infundibular tube. This, then, is the working
hypothesis upon which this book is written. If this view is right, that the
Vertebrate was formed from the Palajostracan without any reversal of surfaces,
but by the amalgamation of the central nervous system and alimentary canal,
then it follows that we have various fixed points of comparison in the central
nervous systems of the two groups of animals from which to search for further
clues. It further follows that from such starting-point every organ of importance
in the body of the arthropod ought to be visible in the corresponding position in
the vertebrate, either as a functional or rudimentary organ. The subsequent
chapters will deal with this detailed comparison of org*ans in the arthropod and
vertebrate respectively.

CHAPTER II

THE EVIDENCE OF THE ORGANS OF VISION

Different kinds of eye. — Simple and compound retinas. —Upright and inverted
retinas. — Median eyes. — Median or pineal eyes of Ammocoetes and their
optic ganglia. — Comparison with other median eyes. — Lateral eyes of verte-
brates compared with lateral eyes of crustaceans. — -Peculiarities of the
lateral eye of the lamprey. — Meaning of the optic diverticula. — Evolution
of vertebrate eyes. — Summary.

The Different Kinds of Eye.

In all animals the, eyes are composed of two parts. 1. A set of
special sensory cells called the retina. 2. A dioptric apparatus for
the purpose of forming an image on the sensory cells. The simplest
eye is formed from a modified patch of the surface-epithelium ; cer-
tain of the hypodermal cells, as they are called, elongate, and their
cuticular surface becomes bulged to form a simple lens. These
elongated cells form the retinal cells, and are connected with the
central nervous system by nerve-fibres which constitute an optic
nerve ; the cells themselves may contain pigment.

The more complicated eyes are modifications of this type for the
purpose of making both tho retina and the dioptric apparatus more
perfect. According to a very prevalent view, these modifications have
been brought about by invaginations of the surface- epithelium.
Thus if ABCD (Fig. 28) represents a portion of the surface-epithelium,
the chitinous cuticle being represented by the dark line, with
the hypodermal cells beneath, and if the part C is modified to form
an optic sense-plate, then an invagination occurring between A and B
will throw the retinal sense-cells with the optic nerve further from
the surface, and the layers B and A between the retina and the source
of light will be available for the formation of the dioptric apparatus.
The lens is now formed from the cuticular surface of A, and the

THE EVIDENCE OF THE ORGANS OF VISION 69

hypodermal cells of A elongate to form the layer known by the name
of corneagen, or vitreogen, the cells of B remaining small and forming
the pre-retinal layer of cells. The large optic nerve end- cells of the
retinal layer, C, take up the position shown in the figure, and their
cuticular surface becomes modified to form rods of varying shape
called rhabdites, which are attached to the retinal cells. Frequently
the rhabdites of neighbouring cells form definite groups, each group
being called a rhabdome. Whatever shape they take it is invariably
found that these little rods (bacilli), or rhabdites, are modifications of
the cuticular surface of the cells which form the retinal layer. Also,
as must necessarily be the case from the method of formation, the
optic nerve arises from the nuclear end of the retinal cells, never from

Fig. 28. — Diagram op Formation op an Upright Simple Retina.

the bacillary end. As in the case first mentioned, so in this case, the
light strikes direct upon the bacillary end of the retinal cells ; such
eyes, therefore, are eyes with an upright retina.

It may happen that the part invaginated is the optic sense-plate
itself, as would be the case if in the former figure, instead of C, the
part B was modified to form a sense-plate. This will give rise to
an eye of a character different from the former (Fig. 29). The optic
nerve- fibres now lie between the source of light and the retinal end-
cells, the layer A as before forms the cuticular lens, and its hypo-
dermal cells elongate to form the corneagen ; there is no pre-retinal
layer, but, on the contrary, a post-retinal layer, C, called the tapetum,
and, as is seen, the light passes through the retinal layer to the

THE ORIGIN OF VERTEBRATES

tapetum. The cuticular surface of the retinal cells forming the rods
or bacilli is directed towards the tapetal layer away from the source of
light, and the nuclei of the retinal cells are pre-bacillary in position,
in contradistinction to the upright eye, where they are post-bacillary.
The retinal end-cells are devoid of pigment, the pigment being in the
tapetal layer.

Such an eye, in contradistinction to the former type, is an eye
with an inverted retina ,• but still the same law holds as in the former
case — the optic nerve-fibres enter at the nuclear ends of the cells,
and the rods are formed from the cuticular surface.

In these eyes the pigmented tapetal layer is believed to act as a
looking-glass ; the dioptric apparatus throws the image on to its

Fig. 29. — Diagram op Formation of an Inverted Simple Retina.

The arrow shows the direction of the source of light in this as in the preceding figure.
In both figures the cuticular rhabdites are represented by thick black lines.

shiny surface, from whence it is reflected directly on to the rods,
which are in close contact with the tapetum. A similar process has
been suggested in the case of the mammalian lateral eye, with its
inverted retina. Johnson describes the post-retinal pigmented layer
as being frequently coloured and shiny, and imagines that it reflects
the image directly back on to the rods.

Thus we see that eyes can be placed in different categories, e.g.
those with an upright retina and those with an inverted retina ; also,
according to the presence or absence of a tapetum, eyes have been
grouped as tapetal or non-tapetal. All the eyes considered so far
are called simple eyes, or ocelli ; and a number of ocelli may be

THE EVIDEXCE OF THE ORGANS OF VISION 7 1

contiguous though separate, as in the lateral eyes of the scorpion.
They may, however, come into close contact and form one single,
large, compound eye. Such ocelli, in a very large number of cases,
retain each its own dioptric apparatus, and therefore the external
appearance of the compound eye represents not a single lens, but a
large number of facets, as is seen in the eyes of insects. Owing to
these differences, eyes have been divided into simple and compound,
and into facetted and non-facetted.

Yet another complication occurs in the formation of eyes, which
is, perhaps, the most important of all : the retinal portion of the eye,
instead of consisting of simple retinal cells, with their accompanying
rhabdites, may include within itself a portion of the central nervous
system.

The rationale of such a formation is as follows : The external
covering of the body is formed by a layer of external epithelial cells
— the ectodermal cell-layer — and an underlying neural layer, of which
the latter gives origin to the central nervous system. As development
proceeds, this central nervous system sinks inwards, leaving as its
connection with the ectoderm the sensory nerves of the skin. That
part of the neural layer which underlies the optic plate forms the
optic ganglion, and when the central nervous system leaves the
surface to take up its deeper position, the strand of nerve-fibres
known as the optic nerve, is left connecting it with the retinal cells
as seen in Figs. 28, 29. It may, however, happen that part of the
optic ganglion remains at the surface, in close connection with the
retinal end-cells, when the rest of the central nervous system sinks
inwards. The retina of such an eye is composed of the combined
optic ganglion and retinal end-cells ; the strand of nerve-fibres which
is left as the connection between it and the rest of the brain, which
is also called the optic nerve, is not a true peripheral nerve, as in
the first case, but rather a tract of fibres connecting two parts of the
brain, of which one has remained at the periphery. Such a retina,
in contradistinction to the first kind, may be called a compound retina.

The optic ganglion, as seen in eyes with a simple retina, consists
of a cortical layer of small, round nerve-cells, and an internal medulla
of fine nerve-fibres, which form a thick network known as 'Punct-
substanz,' or in modern terminology, 'Neuropil.' Fibres which pass
into this 'neuropil' from other parts of the brain connect them
with the optic ganglion.

THE ORIGIN OF VERTEBRATES

At the present time, owing to the researches of Golgi, Ramon y
Cajal, and others, the nervous system is considered to be composed
of a number of separate nerve-units, called neurones, each neurone
consisting of a nerve-cell with its various processes; one of these
— the neuraxon— constitutes the nerve-fibre belonging to that nerve-
cell, the other processes— the dendrites — establish communication
with other neurones. The place where these processes come together
is called a synapse, and the tangle of fine fibres formed at a number
of synapses forms the ' neuropil.'

When, therefore, a compound retina is formed by the amalgama-
tion of the ectodermal part— the retinal cells proper — with the
neurodermic part— to which the name 'retinal ganglion' may be

Pig. 30. — Diagram of Formation of an Upright Compound Retina.

ABCD, as in Fig. 28. Op. g. I. and Op. g. II., two optic ganglia which combine

to form the retinal ganglion, Bt. g.

given, — such a retina consists of neuropil substance and nerve-cells,
as well as the retinal end-cells. In all such compound retinas, the
retinal ganglion is not single, but two optic ganglia at least are
included in it, so that there are two sets of nerve-cells and two
synapses are always formed ; one between the retinal end-cells and
the neurones of the first optic ganglion, which may be called the
ganglion of the retina, the other between the first and second
ganglia, which, seeing that the neuraxons of its cells form the
optic nerve, may be called the ganglion of the optic nerve. The
' neuropil ' formed by these synapses forms the molecular layers of the
compound retina, and the cells themselves form the nuclear layers.
Thus an upright compound retina, formed in the same way as the
upright simple retina, would be illustrated by Fig. 30.

THE EVIDENCE OF THE ORGANS OF VISION J 3

Further, in precisely the same way as in the case of the simple
retina, such a compound retina may be upright or inverted. Thus,
in the lateral eyes of crustaceans and insects, a compound retina of
this kind is formed, which is upright ; while in the vertebrates the
compound retina of the lateral eyes is inverted.

The compound retina of vertebrates is usually described as com-
posed of a series of layers, which may be analyzed into their several
components as follows :—

Layer of rods and cones \

External nuclear layer f retina proper j Ectodermic part

External molecular layer -v

Internal nuclear layer > ganglion of retina

Internal molecular layer ) (retinal j neurodermic

Optic nerve-cell layer \ ganglion of optic nerve ) §anglion ' Part

Layer of optic nerve fibres J

The difference between the development of these two types of
eye — those with a simple retina and those with a compound retina —
has led, in the most natural manner, to the conception that the
retina is developed, in the higher animals, sometimes from the cells of
the peripheral epidermis, sometimes from the tissue of the brain — two
modes of development termed by Balfour 'peripheral' and 'cerebral.'
An historical survey of the question shows most conclusively that all
investigators are agreed in ascribing the origin of the simple retina
to the peripheral method of development, the retina being formed
from the hypodermal cells by a process of invagination, while the
cerebral type of development has been described only in the develop-
ment of the compound retina. The natural conclusion from this fact
is that, in watching the development of the compound retina, it is
more difficult to differentiate the layers formed from the epidermal
retinal cells and those formed from the epidermal optic ganglion-
cells, than in the case of the simple retina, where the latter cells
withdraw entirely from the surface. This is the conclusion to which
Patten has come, and, indeed, judging from the text-book of Kor-
schelt and Heider, it is the generally received opinion of the day
that, as far as the Appendiculata are concerned, the retina, in the
true sense — the retinal end-cells, with their cuticular rods, — is formed,
in all cases, from the peripheral cells of the hypodermal layer, the
cuticular rods being modifications of the general cuticular surface
of the body. The apparent cerebral development of the crustacean

74 THE ORIGIN OF VERTEBRATES

retina, as quoted from Bobretsky by Balfour, is therefore iu reality
the development of the retinal ganglion, and not of the retina proper.
There is, I imagine, a universal belief that the natural mode of
origin of a sense-organ, such as the eye, must always have been from
the cells forming the external surface of the animal, and that direct
origin from the central nervous system is a priori most improbable.
It is, therefore, a matter of satisfaction to find that the evidence for
the latter origin has universally broken down, with the single
exception of the eyes of vertebrates and their degenerated allies ; a
fact which points strongly to the probability that a reconsideration
of the evidence upon which the present teaching of the origin of the
vertebrate eye is based will show that here, too, a confusion has
arisen between that part formed from the epidermal surface and that
from the optic ganglion.

The Median or Pineal Eyes.

Undoubtedly, in recent times, the most important clue to the
ancestry of vertebrates has been given by the discovery that the
so-called pineal gland in the vertebrate brain is all that remains of a
pair of median or pineal eyes, the existence of which is manifest in
the earliest vertebrates ; so that the vertebrate, when it first arose,
possessed a pair of median eyes as well as a pair of lateral eyes.
The ancestor of the vertebrate, therefore, must also have possessed a
pair of median eyes as well as a pair of lateral eyes.

Very instructive, indeed, is the evidence with regard to these
median eyes, for one of the great characteristics of the ancient
palreostracan forms is the invariable presence of a pair of median
eyes as well as a pair of lateral eyes. In the living representative of
such forms — Limulus — the pair of median eyes (Fig. 5) is well
shown, and it is significant that here, according to Lankester and
Bourne, these eyes are already in a condition of degeneration ; so
also in many of the Paheostraca (Fig. 7) the lateral eyes are the large,
well- developed eyes, while the median eyes resemble those of Limulus
in their insignificance.

We see, then, that in the dominant arthropod race at the time
when the fishes first appeared, the type of eyes consisted of a pair of
well-developed lateral eyes and a pair of insignificant, partially
degenerated, median eyes. Further, according to all palaeontologists,

THE EVIDENCE OF THE ORGANS OF VISION 75

in the best-preserved head-shields of the most ancient fishes,
especially well seen in the Osteostraci, in Cephalaspis, Treniataspis,
Auchenaspis, Keraspis, a pair of large, prominent lateral eyes existed,
between which, in the mid-line, are seen a pair of small, insignificant
median eyes.

The evidence of the rocks, therefore, proves that the pair of
median eyes which were originally the principal eyes (Hauptaugen),
had already, in the dominant arthropod group been supplanted by
a pair of lateral eyes, and had, in consequence, become small and
insignificant, at the time when vertebrates first appeared. This dwind-
ling process thus initiated in the arthropod itself has steadily continued
ever since through the whole development of the vertebrates, with the
result that, in the highest vertebrates, these median or pineal eyes
have become converted into the pineal gland with its ' brain-sand.'

In the earliest vertebrate these median eyes may have been
functional ; they certainly were more conspicuous than in later forms.
Alone among living vertebrates the right median eye of Ammoccetes
is so perfect and the skin covering it so transparent that I have
always felt doubtful whether it may not be of use to the animal,
especially when one takes into consideration the undeveloped state
of the lateral eyes in this animal, hidden as they are under the skin.
Thus the one living vertebrate which is comparable with these
extinct fishes is the one in which one of the pineal eyes is most well
defined, most nearly functional.

Before passiDg to the consideration of the structure of the
median eyes of Ammoccetes, it is advisable to see whether these
median eves in other animals, such as arachnids and crustaceans,
belong to any particular type of eyes, for then assuredly the median
eyes of Arnmoccetes ought to belong to the same type if they are
derived from them.

In the specialized crustacean, as in the specialized vertebrate, the
median eyes have disappeared, at all events in the adult, but still
exist in the primitive forms, such as Branchipus, which resemble the
trilobites in some respects. On the other hand, the median eyes
have persisted, and are well developed in the arachnids, both
scorpions and spiders possessing a well- developed pair. The cha-
racteristics of the median eyes must then be especially sought for in
the arachnid group.

Both scorpions and spiders possess many eyes, of which two are

7 6 THE ORIGIN OF VERTEBRATES

always separate and median in position, while the others form lateral
groups ; all these eyes possess a simple retina and a simple corneal
lens. Grenacher was the first to point out that in the spiders two
very distinct types of eye are found. In the one the retina is up-
right ; in the other the retina is inverted, and the eye possesses a
tapetal layer. The distribution of these two types is most suggestive,
for the inverted retina is always found in the lateral eyes, never in
the two median eyes ; these always possess a simple upright retina.

In the crustaceans, the lateral eyes differ also from the median
eyes, but not in the same way as in the arachnids ; for here both
types of eye possess an upright retina, but the retina of the lateral
eyes is compound, while that of the median eyes is simple. In other
words, the median eyes are in all cases eyes with a simple upright
retina and a simple cuticular lens, while the retina of the lateral
eyes is compound or may be inverted, according as the animal
in question possesses crustacean or arachnid affinities. The lateral
eye of the vertebrate, possessing, as it does, an inverted compound
retina, indicates that the vertebrate arose from a stock which was
neither arachnid nor crustacean, but gave rise to both groups — in fact,
was a member of the great palseostracan group. What, then, is the
nature of the median eyes in the vertebrate ?

The Median Eyes of Ammoccetes.

The evidence of Ammoccetes is so conclusive that I, for one, can-
not conceive how it is possible for any zoologist to doubt whether
the parietal organ, as they insist on calling it, had ever been an eye,
or rather a pair of eyes.

Anyone who examines the head of the larval lamprey will see
on the dorsal side, in the median line, first, a somewhat circular orifice
— the unpaired nasal opening ; and then, tailwards to this, a well-
marked circular spot, where the skin is distinctly more transparent
than elsewhere This spot coincides in position with the underlying
dorsal pineal eye, which shines out conspicuously owing to the
glistening wThiteness of its pigment. Upon opening the brain- case
the appearance as in Fig. 20 is seen, and the mass of the right ganglion
habenulce {G.H.R.), as it has been called, stands out conspicuously as
well as the right or dorsal pineal eye (Pn.). Both eye and ganglion
appear at first sight to be one-sided, but further examination shows
that a left ganglion habenulce is present, though much smaller than on

THE EVIDENCE OE THE ORGANS OE VISION

the right side. In connection with this is another eye-like organ — the
left or ventral pineal eye, — much more aborted, much less like an eye
than the dorsal one ; so also there are two bundles of peculiar fibres

Fig. 31. — One op a Series op Horizontal Sections through the Head op

Ammoccetes.

/.;»., upper lip muscles ; m.c, muco-cartilage ; »., nose; na.c, uasal cartilage; pn.,
right pineal eye and nerve; g.h.r., right ganglion habenuhe ; s.m., somatic
muscles; or., membranous wall of cranium; cli., choroid plexus; gl., glandular
substance and pigment filling up brain-case.

called Meynert's bundles, which connect this region with the infra-
infundibular region of the brain ; of these, the right Meynert's bundle

THE ORIGIN OF VERTEBRATES

is much larger than the left. This difference between right and left
indicates a greater degeneration on the left side, and points distinctly
to a close relationship between the nerve-masses known as ganglia
habenulcB and the median eyes. In my opinion this ganglion is, in
part, at all events, the optic ganglion of the median eye on each side.
It is built up on the same type as the optic ganglia of invertebrate

Fig. 32. — Eye op Acilius Larva, with Fig. 33. — Pineal Eye op Ammoccetes,
its Optic Ganglion. with its Ganglion Habenula.

On the right side the nerve end-cells On the left side the eye is drawn as it

have been drawn free from pigment. appeared in the section. On the right

side I have removed the pigment from
the nerve end-cells, and drawn the eye
as, in my opinion, it would appear if
it were functional.

simple eyes, with a cortex of small round cells and a medulla of fine
nerve-fibres. Into this ganglion, on the right side, there passes a very
well-defined nerve — the nerve of the dorsal eye. The eye itself with
its nerve, pn.} and its optic ganglion, g.h.r., is beautifully shown by
means of a horizontal section through the head of Ammoccetes
(Fig. 31). Originally, as described by Scott, the eye stood vertically

THE EVIDENCE OF THE ORGANS OF VISION

-ghr

above its optic ganglion, and presented an appearance remarkably like
Fig. 32, which represents one of the simple eyes and optic ganglia
of a larva of Acilius as described by Patten ; then, with the forward
growth of the upper lip,
the right pineal eye was
dragged forward and its
nerve pulled horizon-
tally over the ganglion
habenulce. For this
reason the eye, nerve,
and ganglion are better
shown in a nearly hori-
zontal than in a trans-
verse section.

The optic nerve be-
longing to this eye is
most evident and clearly
shown in Fig. 31, and in
the series of consecutive
sections which follow
upon this section ; no
doubt can arise as to
the structure in ques-
tion having been the
nerve of the eye, even
though, as is possible, it
does not contain any
functional nerve-fibres.

The second, ventral
or left, eye, belonging
to the left ganglion
habenuhe is very dif-
ferent in appearance,
being much less evi-
dently an eye. Fig. 34
is one of the same

pn.

Fig. 34.— Horizontal Section through Brain of
Ammoccetes, to show the Left, or Ventral
Pineal Eye.

, left or ventral pineal eye ; pn.u last remnant of
right, or dorsal pineal eye ; g.h.r., right ganglion
habenulce; g.h.l.lt g.h.l.3, parts of left ganglion
habenulce ; pi., fold oipia mater which separates
the left ganglion habenulce from the left pineal
eye ; /., strands of nerve-fibres connecting the
left eye with its ganglion, g.h.l.3; V3, third

ventricle; Y.aq., ventricle of aquseduct.

series of horizontal sections as Fig. Sl,pn.i being the last remnant
of the right, or dorsal, eye, while pn.% shows the left, or ventral, eye
with its connection with the left ganglion habenulce.

80 THE ORIGIN OF VERTEBRATES

In a series of sections I have followed the nerve of the right pineal
eye to its destination, as described in my paper in the Quarterly
Journal of Microscopical Science, and have found that it enters into
the ganglion habenulce just as the nerve to any simple eye enters
into its optic ganglion. This nerve, as I have shown, is a very dis-
tinct, well-defined nerve, with no admixture of ganglion-cells or of
connective tissue, very different indeed to the connection between
the left pineal eye and its optic ganglion. Here there is no denned
nerve at all ; but the cells of the ganglion habenulce stretch right up to
the remains of the eye itself. Seeing, then, that both the eye and
ganglion on this side have reached a much further grade of degenera-
tion than on the right side, it may be fairly concluded that the
original condition of these two median eyes is more nearly repre-
sented by the right eye, with its well-defined nerve and optic gang-
lion, than by the left eye, or by the eyes in lizards and other animals
which do not show so well-defined a nerve as is possessed by
Ammoccetes. Quite recently Dendy has examined the two median
eyes in the New Zealand lamprey Gcotria australis. In this species
the second eye is much better defined than in the European lamprey,
and its connection with the ganglion habenulce is more nerve-like.
In neither eye, however, is the nerve so clean cut and isolated as is the
nerve of the dorsal, or right, eye in the Ammoccetes stage of Petromy-
zon Planeri; in both, cells resembling those of the cortex of the
ganglion habenulce and connective tissues are mixed up with the
nerve-fibres which pass from each eye to its respective optic ganglion.

The Eight Pineal Eye of Ammoccetes.

The optic fibres of the right median eye of Ammoccetes are con-
nected with a well-defined retina, the limits of which are defined
by the white pigment so characteristic of this eye. This pigment is
apparently calcium phosphate, which still remains as the ' brain-sand '
of the human pineal gland. The cells, which are hidden by this pig-
ment, were described by me in 1890 as the retinal end-cells with large
nuclei. In 1893, Studnicka examined them more closely, and con-
cluded that the retinal cells are of two kinds : the one, nerve end-cells,
the sensory cells proper ; the other, pigmented epithelial cells, which
surround the sense- cells. The sense-cells may contain some of the
white pigment, but not so much as the other cells. Similarly, in the

THE EVIDENCE OF THE ORGANS OF VISION 8 1

median eyes of Limulus, Lankester and Bourne find it difficult to
determine how far the retinal end-cells contain pigment and how far
that pigment really is in the cells surrounding these nerve end-cells.

The interior of the eye presents the appearance of a cavity in
shape like a cornucopia, the stalk of which terminates at the place
where the nerve enters. This cavity is not empty, but the posterior
part of it is filled with the termination of the nerve end-cells of the
retina, as pointed out by me and confirmed by Studnicka. These
terminations are free from pigment, and contain strikingly trans-
lucent bodies, which I have described in my paper in the Quarterly
Journal, and called rhabdites, for they present the same appearance
and are situated in the same position as are many of the rhabdites
on the terminations of the retinal end-cells of arthropod eyes.
Studnicka has also seen these appearances, and figures them in
his second paper on the nerve end-cells of the pineal eye of
Ammoccetes.

Up to this point the following conclusions may be drawn : —

1. Ammoccetes possesses a pair of median eyes, just as was the

case with the most ancient fishes, and with the members of
the contemporary paheostracan group.

2. The retina of one of these eyes is well-defined and upright,

not inverted, and therefore in this respect agrees with that
of all median eyes.

3. The presence of nerve end-cells, with pigment either in them or

in cells around them, to the unpigmented ends of which trans-
lucent bodies resembling rhabdites are attached, is another
proof that this retina agrees with that of the median eyes of
arthropods.

4. The simple nature of the nerve with its termination in an

optic ganglion closely resembling in structure an arthropod

optic ganglion, together with Studnicka' s statement that the

nerve end-cells pass directly into the nerve, points directly

to the conclusion that this retina is a simple, not a compound,

retina, and that it therefore in this respect also agrees with

the retina of all median eyes.

With respect to this last conclusion, neither I myself nor

Studnicka have been able to see any definite groups of cells

between the nerve end-cells and the optic nerve such as a compound

retina necessitates.

82 THE ORIGIN OF VERTEBRATES

On the other hand, Dently describes in the New Zealand lamprey,
Gcotria australis, a cavity where the nerve enters into the eye,
which he calls the atrium. This cavity is distinct from the main
cavity of the eye, and is separated from it by a mass of cells similar
in appearance to those of the cortex of the ganglion hahcnulcc. In
these two eyes then, groups of cells, resembling in appearance those
belonging to an optic ganglion, exist in the eyes themselves. This
atrium is evidently that part of the central cavity which I have
called the handle of the cornucopia in the European lamprey, and
the very fact that it is separated from the rest of the central cavity
is evidence that we are dealing here with a later stage in the history
of the pineal eyes than in the case of the Ammoccetes of Petromijzon
Planeri. Taking also into consideration the continuity of the mass
of small ganglion-cells which surround this atrium with the cells of
the ganglion habcnulce by means of the similar cells scattered along
the course of the nerve, and also bearing in mind the fact already
stated that in the more degenerate left eye of Ammocoetes the cells
of the ganglion habenulce extend right up to the eye itself, it seems
more likely than not that these cells do not represent the original
optic ganglion of a compound retina, but rather the subsequent
invasion, by way of the pineal nerve, of ganglion-cells belonging to
a portion of the brain. In the last chapter it has been suggested
that the presence of the trochlear or fourth cranial nerve has given
rise to the formation of the cerebellum by a similar spreading.

There is certainly no appearance in the least resembling a
compound retina such as is seen in the vertebrate or crustacean
lateral eye. In the median eyes of scorpions and of Limulus, cells
are found within the capsule of the eye among the nerve-fibres and
the nerve end-cells. These are especially numerous in the median
eyes of Limulus, as described by Lankester and Bourne, and are
called by them intrusive connective tissue cells. The meaning of
these cells is not, to my mind, yet settled. It is sufficient for my
purpose to point out that the presence of cells interneural in position
among the nerve end-cells of the retina of the median eyes of
Ammoccetes is more probable than not, on the assumption that the
retina of these eyes is built up on the same plan as that of the
median eyes in Limulus and the scorpions.

It is further to be borne in mind that these specimens of Gcotria
worked at by Dendy were in the ' Velasia ' stage of the New Zealand

THE EVIDENCE OF THE ORGANS OF VISION

lamprey, and correspond, therefore, more nearly to the l'etromyzon
than to the Ammoccetes stage of the European lamprey.

The Dioptric Apparatus.

Besides the retina, all eyes possess a dioptric apparatus. What
is the evidence as to its nature in these vertebrate median eyes ?
Lankester and Bourne have divided the eyes of scorpions and

Til

ret

Fig. 35. — Eye of Acilius Lakv^e. (After Patten.)

I., chitinous lens ; c, corneagen; pr., pre-retinal layer ; rlu, rhabdites ; ret., retinal

end-cells.

Limulus into two kinds, monostichous and diplostichous. In the
first the retinal cells are supposed to give rise to not only rhabdites
but also the cuticular chitinous lens, so that the eye is one-layered ;
in the second the lens is formed by a well-marked hypodermal layer,
in front of the retina, composed of elongated cells, so that these eyes
are two-layered or diplostichous. The lateral eyes, according to
them, are all monostichous, but the median eyes are diplostichous.
This distinction is not considered valid by other observers. Thus,

THE ORIGIN OF VERTEBRATES

as already indicated, Patten looks on all these eyes as three-layered,
and states that in all cases a corneagen or vitreogen layer exists,
which gives origin to the lens. For my own part I agree with

Patten, but we are not con-
cerned here with the lateral
eyes. It is sufficient to note
that all observers are agreed
that the median eyes are
characterized by this well -
marked cell-layer, the so-called
vitreous or corneagen layer of
cells.

This layer (p., Fig. 35) is
composed of much - elongated
cells of the hypodermal layer,
in each of which the large
nucleus is always situated to-
wards the base of the cell.
The space between it and the

Fig. 36. — Eye op Hydrophilus Larva,
with the Pigment over the Retinal
End-cells.

retina contains, according to

I., chitinous lens; c, corneagen; pr., pre-
retinal layer ; rh., rhabdites; ret., retinal

end-cells.

Patten the cells of the pre-
retinal layer (pr.). These may be so few and insignificant as to give
the impression that the vitreous layer is immediately adjacent to the
retina (ret.).

Let us turn now to the right pineal eye of Ammoccetes (Fig. 37)
and see what its further structure is. The anterior part of the eye
is free from pigment, and is composed, as is seen in hsematoxylin or
carmine specimens, of an inner layer of nuclei which are frequently
arranged in a wavy line. From this nucleated layer, strands of tissue,
free from nuclei, pass to the anterior edge of the eye.

In the horizontal longitudinal sections it is seen that these strands
are confined to the middle of the eye. On each side of them the
nuclear layer reaches the periphery, so that if we consider these
strands to represent long cylindrical cells, as described by Beard,
then the anterior wall may be described as consisting of long
cylindrical cells, which are flanked on either side by shorter cells of
a similar kind. The nuclei at the base of these cylindrical cells are
not all alike. We see, in the first place, large nuclei resembling the
large nuclei belonging to the nerve end-cells ; these are the nuclei of

THE EVIDENCE OF THE ORGANS OF VISION

the long cylindrical cells. We see also smaller nuclei in among
these larger ones, which look like nuclei of intrusive connective
tissue, or may perhaps form a distinct layer of cells, situated between
the cells of the anterior wall and the terminations of the nerve
end- cells already referred to.

These elongated cells are in exactly the same position and present
the same appearance as the cells of the corneagen layer of any median

eye. Like the latter they are

free from pigment and never
show with osmic staining any
sign of the presence of trans-
lucent rhabdite - like bodies,
such as are seen in the termi-
nation of the retinal cells, and
like the latter their nuclei are
at the base. The resemblance
between this layer and the
corneagen cells of any median
eye is absolute. Between it
and the terminations of the
retinal cells there exists some
ill-defined material certainly
containing cells which may
well correspond to Patten's
pre-retinal layer of cells.

Eetina, corneagen, nerve,
optic ganglion, all are there, all
in their right position, all of
the right structure, what more
is needed to complete the
picture ?

In order to complete the dioptric apparatus a lens is necessary.
Where, then, is the lens in these pineal eyes ? In all the arachnid eyes,
whether median or lateral, the lens is a single corneal lens composed
of the external cuticle, which is thickened over the corneagen cells.
This thickened cuticle is composed of chitin, and is not cellular,
but is dead material formed out of the living underlying corneagen
cells. Such a lens is in marked contrast to the lens of the lateral
vertebrate eye, which is formed by living cells themselves. This

Fig. 37. — Pineal Eye of Ammoccetes,
with its Ganglion Habenulcz.

86 THE ORIGIN OF VERTEBRATES

thickening of the cnticnlar layer to form a lens could only exist as
long as that layer is absolutely external, so that the light strikes
immediately upon it ; for, if from any cause the eye became situated
internally, the place of such a lens must be filled by the structures
situated between it and the surface, and the thickened cuticle would
no longer lie formed.

In all vertebrates these pineal eyes are separated from the
external surface by a greater or less thickness of tissues ; in the
case of Ammoccetes, as is seen in Fig. 31, the eye lies within the
membranous cranial wall, and is attached closely to it. The position,
then, of the cuticular, or corneal lens, as it is often called, on the
supposition that this is a median eye of the arachnid type, is taken
by the membranous cranium, and, as I have described in my
paper in the Quarterly Journal, on carefully lifting the eye in the
fresh condition from the cranial wall, it can be seen under a
dissecting microscope that the cranial wall often forms at this
spot a lens-like bulging, which fits the shallow concavity of the
surface of the eye, and requires some little force to separate it from
the eye.

As will appear in a subsequent chapter, this cranial wall has
been formed by the growth, laterally and dorsally, of a skeletal
structure known by the name of the plastron. The last part of it to
be completed would be that part in the mid-dorsal line, where appa-
rently, in consequence of the insinking of the degenerating eyes, a
dermal and subdermal layer already intervened between the source
of light and the eyes themselves.

When the membranous cranium was completed in the mid-dorsal
region, it was situated here as elsewhere just internally to the sub-
dermal layer, and therefore enclosed the pineal eyes. This, to my
mind, is the reason why the pineal eyes, which, in all other respects,
conform to the type of the median eyes of an arachnid-like animal,
do not possess a cuticular lens. Other observers have attempted to
make a lens out of the elongated cells of the anterior wall of the
eye (my corneagen layer), but without success.

Studnicka, who calls this layer the pellucida, does not look upon
it as the lens, but considers, strangely enough, that the translucent
appearances at the ends of each nerve end-cell represent a lens for
that cell, so that every nerve end-cell has its own lens. Still more
strange is it that, holding this view, he should yet consider these knobs

THE EVIDENCE OF THE ORGANS OF VISION 87

to be joined by filaments to the cells in the anterior wall of the eye,
a conception fatal to the action of such knobs as lenses.

The discovery that the vertebrate possesses, in addition to the
lateral eyes, a pair of median eyes, which are most conspicuous in
the lowest living vertebrate, together with the fact that such eyes
are built up on the same plan as the median eyes of living crus-
taceans or arachnids, not only with respect to the eye itself but also
to its nerve and optic ganglion, constitutes a fact of the very greatest
importance for any theory of the origin of vertebrates ; especially in
view of the further fact, that similar eyes in the same position are
found not only in all the members of the Palaaostraca, but also in all
those ancient forms (classed as fishes) which lived at that time. At
one and the same moment it proves the utter impossibility of
reversing dorsal and ventral surfaces, points in the very strongest
manner to the origin of the vertebrate from some member or other
of the paloeostracan group, and insists that the advocates of the
origin of vertebrates from the Hemichordata, etc., should give an
explanation of the presence of these two median eyes of a more con-
vincing character than that given here.

The Lateral Eyes.

Turning now to the consideration of the lateral eyes, we see that
these eyes in the arachnids often possess an inverted retina, in the
crustaceans always an upright retina. In the arachnids they possess
a simple retina, while in the crustaceans their retina is compound ;
so that in the latter case the so-called optic nerve is in reality a
tract of fibres connecting together the brain-region with a variable
number of optic ganglia, which have been left at the periphery in
close contact with the retinal cells, when the brain sunk away from
the superficial epithelial covering.

There is, then, this difference between the lateral eyes of crus-
taceans and arachnids, that the retina of the former is compound, but
never inverted, while that of the latter may be inverted, but is
always simple.

The retina of the lateral eyes of the vertebrate resembles both of
these, for it is compound, as in the crustacean, and inverted as in
the arachnid.

It must always be borne in mind that in the palreostracan epoch

88 THE ORIGIN OF VERTEBRATES

the dominant race was neither crustacean nor arachnid, but partook
of the characters of both ; also, as is characteristic of dominance,
there was very great variety of form, so that it seems more probable
than not that some of these forms may have combined the arachnid
and crustacean characteristics to the extent of possessing lateral eyes
with an inverted yet compound retina. A certain amount of
evidence points in this direction. As already stated, the compound
retina which characterizes the vertebrate lateral eyes is character-
istic of all facetted eyes, and in the trilobites facetted lateral eyes
are commonly found. From this it may be concluded that many of
the trilobites possessed eyes with a compound retina. There have,
however, been found in certain species, e.g. Harpcs vittatus and
Harpes ungula, lateral eyes which were not facetted, and are believed
by Korschelt and Heider to be of an arachnid nature. They say,
" Palaeontologists have appropriately described them as ocelli,
although, from a zoological point of view, they do not deserve this
name, having most probably arisen in a way similar to that con-
jectured in connection with the lateral eyes of scorpions." If this
conjecture is right, then in these forms the retina may have been
inverted, but because they belonged to the trilobite group, the retina
was most probably compound, so that here we may have had the
combination of the arachnid and crustacean characteristics. On the
other hand, in some forms of Branchipus, and many of the Gamma-
ridse, a single corneal lens is found in conjunction with an eye of the
crustacean type, so that a non-facetted lateral eye, found in a fossil
form, would not necessarily imply the arachnid type of eye with the
possibility of an inverted retina. Whatever may lie the ultimate
decision upon these particular forms, the striking fact remains, that
both in the vertebrate and in the arachnid the median eyes possess
a simple upright retina, while the lateral eyes possess an inverted
retina, and that both in the vertebrate and the crustacean the
median eyes possess a simple upright retina, while the lateral eyes
possess a compound retina.

The resemblance of the retina of the lateral eyes of vertebrates
to that of the lateral eyes of many arthropods, especially crustaceans,
has been pointed out by nearly every one who has worked at these
invertebrate lateral eyes. The foundation of our knowledge of the
compound retina is Berger's well-known paper, the results of which
are summed up by him in the following two main conclusions.

THE EVIDENCE OF THE ORGANS OF VISION

1. The optic ganglion of the Arthropoda consists of two parts, of
which the one stands in direct inseparable connection with the
facetted eye, and together with the layer of retinal rods forms the
retina of the facetted eye, while the other part is connected rather
with the brain, and is to be considered as an integral part of the
brain in the narrower sense of the word.

2. In all arthropods examined by him, the retina consists of five
layers, as follows :—

(1) The layer of rods and their nuclei.

(2) The layer of nerve-bundles.

(3) The nuclear layer.

(4) The molecular layer.

(5) The ganglion cell layer.

Berger passes under review the structure and arrangement of
the optic ganglion in a large number of different groups of arthropods,
and concludes that in
all cases one part of
the optic ganglion is
always closely attached
to the visual end-cells,
and this combination
he calls the retina.
On the other hand, the
nerve-fibres which con-
nect the peripheral part
of the optic ganglion
with the brain, the so-
called optic nerve, are
by no means homolo-
gous in the different
groups ; for in some
cases, as in many of
the stalk-eyed crusta-
ceans, the whole optic
ganglion is at the pe-
riphery, while in others, as in the Diptera, only the retinal ganglion
is at the periphery, and the nerve-stalk connects this with the rest
of the optic ganglion, the latter being fused with the main brain-
mass. In the Diptera, in fact, according to Berger, the optic nerve

Fig. 38.— The Retina of Musca. (After Berger.)

Br., brain; O.n., optic nerve; n.l.o.g., nuclear layer of
ganglion of optic nerve; m.L, molecular layer
(Punktsubstanz) ; n.l.r.g.i. and n.l.r.g.o., inner and
outer nuclear layers of tbe ganglion of tbe retina ;
f.lr.r., terminal fibre-layer of retina; r., layer of
retinal end-cells (indicated only).

THE ORIGIN OF VERTEBRATES

and retina are most nearly comparable to those of the vertebrate.
For this reason I give Berger's picture of the retina of Musca
(Fig. 38), in order to show the arrangement there of the retinal
layers.

In Branchipus and other primitive Crustacea, Berger also finds
the same retinal layers, but is unable to distinguish in the brain the
rest of the optic ganglion. Judging from Berger's description of
Branchipus, and Bellonci's of Spha^roma, it would almost appear
as though the cerebral part of the retina in the higher forms
originated from two ganglionic enlargements, an external and

Sup. Segment Ant I

Ant II
Inf. Segment

Fig. 39. — The Brain op Sphceroma scrratum. (After Bellonci.)

Ant. I. and Ant. II., nerves to 1st and 2nd antennae, f.br.r., terminal fibre-layer of
retina; Op. g. I., first optic ganglion; Op. g. II., second optic ganglion; O.n.,
optic nerve-fibres forming an optic chiasma.

internal enlargement, as Bellonci calls them. The external ganglion
(Op. g. I., Fig. 39) may be called the ganglion of the retina, the cells
of which form the nuclear layer of the higher forms, and the internal
ganglion (Op. g. II., Fig. 39), from which the optic nerve-fibres to the
brain arise, may therefore be called the ganglion of the optic nerve.
Bellonci describes how in this latter ganglion cells are found very
different to the small ones of the external ganglion or ganglion of
the retina. So also in Branchipus, judging from the pictures of
Berger, Claus, and from my own observations (ef. Fig. 46, in which
the double nature of the retinal ganglion is indicated), the peripheral
part of the optic ganglion — i.e. the retinal ganglion — may be spoken

THE EVIDENCE OF THE ORGANS OF VISION

f.br.r
b.rn

nl.r.g,

immm

$?— ml

of as composed of two ganglia. The external of these is clearly the
ganglion of the retina ; its cells form the nuclear layer, the striking
character of which, and close resemblance to the corresponding layer
in vertebrates, is shown by Claus' picture, which I reproduce (Fig. 40).
The internal ganglion with which the optic nerve is in connection
contains large ganglion cells, which, to-
gether with smaller ones, form the gang-
lionic layer of Berger.

The most recent observations of the
structure of the compound retina of the
crustacean eye are those of Parker, who,
by the use of the methylene blue method,
and Golgi's method of staining, has been
able to follow out the structure of the
compound retina in the arthropod on the
same lines as had already been done for
the vertebrate. These two methods have
led to the conclusion that the arthropod
central nervous system and the verte-
brate central nervous system are built up
in the same manner — viz. by means of a
series of ganglia connected together, each
ganglion being composed of nerve-cells,
nerve-fibres, and a fine reticulated sub-
stance called by Leydig in arthropods
' Punktsubstanz,' and known in verte-
brates and in invertebrates at the present
time as 'neuropil.' A further analysis
resolves the whole system into a combi-
nation of groups of neurones, the cells
and fibres of which form the cells and
fibres of the ganglia, while their dendritic
connections with the terminations of other neurones, together with
the neuroglia-cells form the 'neuropil.' As is natural to expect,
that part of the central nervous system which helps to form the
compound retina is built up in the same manner as the rest of the
central nervous system.

Thus, according to Parker, the mass of nervous tissue which
occupies the central part of the optic stalk in Astacus is composed

Fig. 40. — Bipolar Cells of
Nuclear Layer in Ketina
op Branchipus. (After
Claus.)

f.br.r,, terminal fibre - layer
of retina; n.l.r.g., bipolar
cells of tbe ganglion of the
retina = inner nuclear layer ;
m.l., Punktsubstanz = inner
molecular layer ; b.m., base-
ment membrane formed by
neurilemma round central
nervous system.

92 THE ORIGIN OF VERTEBRATES

of four distinct ganglia; the retina is connected with the first of
these by means of the retinal fibres, and the optic nerve extends
proximally from the fourth ganglion to the brain. Each ganglion con-
sists of ganglion-cells, nerve-fibres, and 'neuropil/ and, in addition,
supporting cells of a neuroglial type. By means of the methylene
blue method and the Golgi method, it is seen that the retinal end-
cells, with their visual rods, are connected with the fibres of the
optic nerve by means of a system of neurones, the synapses of
which take place in and help to form the ' neuropil ' of the various
ganglia. Thus, an impulse in passing from the retina to the brain
would ordinarily travel over five neurones, beginning with one of
the first order and ending with one of the fifth. He makes five
neurones although there are only four ganglia, because he reckons
the retinal cell with its elongated fibre as a neurone of the first
order, such fibre terminating in dendritic processes which form
synapses in the ' neuropil ' of the first ganglion with the neurones of
the second order.

Similarly the neurones of the second order terminate in the
' neuropil ' of the second ganglion, and so on, until we reach the
neurones of the fifth order, which terminate on the one hand in the
' neuropil ' of the fourth ganglion, and on the other pass to the optic
lobes of the brain by their long neuraxons — the fibres of the optic
nerve.

He compares this arrangement with that of Branchipus, Apus,
Estheria, Daphnia, etc., and shows that in the more primitive
crustaceans the peripheral optic apparatus was composed, not of
four but of two optic ganglia, not, therefore, of five but of three
neurones, viz. —

1. The neurone of the first order — i.e. the retinal cell with its
fibre terminating in the ' neuropil ' of the first optic ganglion (ganglion
of the retina).

2. The neurone of the second order, which terminates in the
' neuropil ' of the second ganglion (ganglion of the optic nerve).

3. The neurone of the third order, which terminates in the optic
lobes of the brain by means of its neuraxons (the optic nerve).

We see, then, that the most recent researches agree with the
older ones of Berger, Claus, and Bellonci, in picturing the retina of
the primitive crustacean forms as formed of two ganglia only, and
not of four, as in the specialized crustacean group the Malacostraca.

THE EVIDENCE OF THE ORGANS OF VISION 93

The comparison of the arthropod compound retina with that of
the vertebrate shows, as one would expect upon the theory of the
origin of vertebrates put forward in this book, that the latter retina
is built up of two ganglia, as in the more primitive less specialized
crustacean forms. The modern description of the vertebrate retina,
based upon the Golgi method of staining, is exactly Parker's descrip-
tion of the simpler form of crustacean retina in which the ' neuropil '
of the first ganglion is represented by the external molecular
layer, and that of the second ganglion by the internal molecular
layer ; the three sets of neurones being, according to Parker's
terminology : —

1. The neurones of the first order — viz. the visual cells — the
nuclei of which form the external nuclear layer, and their long
attenuated processes form synapses in the external molecular layer
with

2. The neurones of the second order, the cells of which form the
internal nuclear layer, and their processes form synapses in the
internal molecular layer with

3. The neurones of the third order, the cells of which form the
ganglionic layer and their neuraxons constitute the fibres of the optic
nerve which end in the optic lobes of the brain.

Strictly speaking, of course, the visual cells with their elongated
processes have no right to be called neurones : I only use Parker's
phraseology in order to show how closely the two retinas agree even
to the formation of synapses between the fine drawn-out processes of
the visual cells and the neurones of the ganglion of the retina.

The Eetina of the Lateral Eye of Ammoccetes.

As in the case of all other organs, it follows that if we are dealing
here with a true genetic relationship, then the lower we go in the
vertebrate kingdom the more nearly ought the structure of the retina
to approach the arthropod type. It is therefore a matter of intense
interest to determine the nature of the retina in Ammoccctes in order
to see whether it differs from that of the higher vertebrates, and if
so, whether such differences are explicable by reference to the structure
of the arthropod eye.

Before describing the structure of this retina it is necessary to

clear away a remarkable misconception, shared among others by

94 THE ORIGIN OF VERTEBRATES

Balfour, that this eye is an aborted eye, and that it cannot be
considered as a primitive type. Thus Balfour says : " Considering
the degraded character of the Ammoccete eye, evidence derived from
its structure must be received with caution," and later on, "the most
interesting cases of partial degeneration are those of Myxine and the
Ammoccete. The development of such aborted eyes has as yet been
studied only in the Ammoccete, in which it resembles in most
important features that of other Vertebrata."

Again and again the aborted character of the eye is stated to be
evidence of degeneration in the case of the lamprey. What such a
statement means, why the eye is in any way to be considered as
aborted, is to me a matter of absolute wonderment : it is true that
in the larval form it lies under the skin, but it is equally true that
at transformation it comes to the surface, and is most evidently as
perfect an eye as could be desired. There is not the slightest sign
of any degeneration or abortion, but simply of normal development,
which takes a longer time than usual, lasting as it does throughout
the life-time of the larval form.

Kohl, who has especially studied degenerated vertebrate eyes,
discusses with considerable fulness the question of the Ammocoetes
eye, and concludes that in aborted eyes a retarded development
occurs, and this applies on the whole to Ammocoetes, " but with the
important difference that in this case the period of retarded develop-
ment is not followed by a stoppage, but on the contrary by a period
of very highly intensified progressive development during the meta-
morphosis," with the result that " the adult eye of Petromyzon
Planeri does not diverge from the ordinary type."

Eeferring in his summing up to this retarded development, he
says : " Such reminiscences of embryonic conditions are after all
present here and there in normally developed organs, and by no
means entitle us to speak of abnormal development."

The evidence, then, is quite clear that the eye of Petromyzon,
or, indeed, of the full-grown Ammocoetes, is in no sense an abnormal
eye, but simply that its development is slow during the animoccete
stage. The retina of Petromyzon was figured and described by
Langerhans in 1873. He describes it as composed of the following
layers : —

(1) Membrana limitans interna.

(2) Thick inner molecular layer.

THE EVIDENCE OF THE ORGANS OF VISION

(3) Optic fibre layer.

(4) Thick inner nuclear layer.

(5) Peculiar double-layered ganglionic layer.

(6) External molecular layer.

(7) External nuclear layer.

(8) Mcmbrana limitans externa.

(9) Layer of rods.

(10) Pigment-epithelium.
He points out especially the peculiarity of layer (2) (2, Eig. 41), the
inner molecular, in which two rows of nuclei are arranged with great

Fig. 41. — Retina and Optic Nerve of Petromyzon. (After Muller and

Langerhans.)

On the left side the Mullerian fibres and pigment-epithelium are represented alone.
The retina is divided into an epithelial part, C (the layer of visual rod-cells), and
a neurodermal or cerebral part which is formed of, A, the gauglion of the optic
nerve and, B, the ganglion of the retina. 1, int. limiting membrane ; 2, int.
molecular layer with its two layers of cells ; 3, layer of optic nerve fibres ; 4, int.
nuclear layer ; 5, double row of tangential fulcrum cells ; 6, layer of terminal
retinal fibres; 7, ext. nuclear layer; 8, ext. limiting membrane; 9, layer of
rods ; 10, layer of pigment-epithelium. D, axial cell layer (Axenstrang) in optic
nerve. The layer 6 is drawn rather too thick.

regularity, the one row closely touching the mcmbrana limitans
interna, the other at the inner boundary of the middle third of the

96 THE ORIGIN OF VERTEBRATES

molecular layer. Of these two rows of nuclei, he describes the inner-
most as composed almost entirely of large nuclei belonging to ganglion
cells, while the outermost is composed mainly of distinctly smaller
nuclei, which in staining and appearance appear to belong not to
nerve-cells but to the true reticular tissue of the molecular layer.

He also draws special attention to the remarkable layer (5) (5,
Fig. 41), which is not found in the retina of the higher vertebrates,
the cells of which, in his opinion, are of the nature of ganglion-cells.

W. Miiller, in 1874, gave a most careful description of the eye
of Ammoccetes and Petromyzon, and traced the development of the
retina; the subsequent paper of Kohl does not add anything new,
and his drawings are manifestly diagrams, and do not represent the
appearances so accurately as Miiller's illustrations. In the
accompanying figure (Fig. 41) I reproduce on the right-hand side
Miiller's picture of the retina of Petromyzon, but have drawn it, as
in Langerhans' picture, at the place of entry of the optic nerve.

From his comparison of this retina with a large number of other
vertebrate retinas, he comes to the conclusion that the retina of all
vertebrates is divisible into

A. An ectodermal (epithelial) part consisting of the layer of the

visual cells, and

B. A neuroclermal (cerebral) part which forms the rest of the

retina.
Further, Miiller points out that the neuroderm gives origin through-
out the central nervous system to two totally different structures, on
the one hand to the true nervous elements, on the other to a system
of supporting cells and fibres which cannot be classed as connective
tissue, for they do not arise from mesoblast, and are therefore called
by him ' fulcrum-cells.' In the retina he recognizes two distinct
groups of such supporting structures — (1) a system of radial fibres
with well-marked elongated nuclei, which extend between the two
limiting layers, and form at their outer ends a membrane-like
expansion which was originally the outer limit of the retina, but
becomes afterwards co-terminous with the mcmbrana limitans
externa, owing to the piercing through it of the external limbs of the
rods. This system, which is known by the name of the radial
Miillerian fibres (shown on the left-hand side of Fig. 41), has no
connection with (2) the spongioblasts and neurospongium, which
form a framework of neuroglia, in which the terminations of the

THE EVIDENCE OF THE ORGANS OF VISION 97

optic ganglion and of the retinal ganglion ramify to form the mole-
cular layers.

It is evident from Fig. 41 that the retina of Ammoccetes and
Petromyzon differs in a striking manner from the typical vertebrate
retina. The epithelial part (C) remains the same — viz. the visual
rods, the external limiting membrane, and the external nuclear
layer; but the cerebral part, the retinal ganglion (A and B), is
remarkably different. It is true, it consists in the main of the
small-celled mass known as the inner nuclear layer, and of the
reticulated tissue or ' neuropil ' known as the inner molecular layer,
just as in all other compound retinal eyes; but neither the ganglion
cell-layer nor the optic fibre-layer is clearly defined as separate from
this molecular layer ; on the contrary, it is matter of dispute as to
what cells represent the ganglionic layer of higher vertebrates, and
the optic fibres do not form a distinct innermost layer, but pass into
the inner molecular layer at its junction with the inner nuclear
layer. A comparison of this innermost part of the retina (A, Fig.
41), with the corresponding part in Berger's picture of Musca {n.l.o.g.,
Fig. 38), shows a most striking similarity between the two. In both
cases the fibres of the optic nerve (O.n., Fig. 38) which cross at their
entrance pass into the ' neuropil ' of this part of the retinal ganglion,
and are connected probably (though that is not proved in either
case) with the cells of the ganglionic layer. In both cases we find
two well-marked parallel rows of cells in this part of the retina, of
which one, the innermost, is composed in Ammoccetes of large
ganglion-cells, and the other mainly of smaller, deeper staining cells
apparently supporting in function. Similarly, also, in Branchipus, as
I conclude from my own observations as well as from those of Berger
and Claus, the ganglionic layer is composed partly of true ganglion-
cells and partly of supporting cells arranged in a distinct layer. This
part, then, of the retina of Ammoccetes is remarkably like that of a
typical arthropod retina, and forms that part of the retinal ganglion
which may be called the ganglion of the optic nerve.

Next comes the ganglion of the retina (B, Fig. 41) (Parker's first
optic ganglion), the cells of which form the small bipolar granule-
cells of the inner nuclear layer; granule-cells arranged in rows just
as they are shown in Claus' picture of the same layer in the retina
of Branchipus (Fig. 40), just as they are found in the cortical layers
of the optic ganglion of the pineal eye (ganglion habcnulcr), in the

9§ THE ORIGIN OF VERTEBRATES

optic lobes and other parts of the Ammoccetes brain, or in the cortical
layers of the optic ganglia of all arthropods.

Between this small-celled nuclear layer (4, Fig. 41) and the layer
of nuclei of the visual rod cells (7, Fig. 41) (the external nuclear
layer), we find in the eye of Ammoccetes and Petromyzon two well-
marked rows of cells of a most striking character — viz. the two
remarkably regular rows of large epithelial-like cells with large
conspicuous nuclei, which give the appearance of two opposing rows
of limiting epithelium (5, Fig. 41), already mentioned in connection
with the researches of Langerhans and W. Miiller. Here, then, is a
striking peculiarity of the retina of the lamprey, and according to
Miiller the obliteration of these two layers can be traced as we pass
upwards in the vertebrate kingdom. Among fishes, they are especially
well seen in the perch ; in the higher vertebrates the whole layer is
only a rudiment represented, he thinks, by the simple layer of round
cells which lies close against the inner surface of the layer of
terminal fibres (Nervenansatze), and is especially evident in birds
and reptiles. In man and the higher mammals they are probably
represented by the horizontal cells of the outer part of the inner
nuclear layer.

Seeing, then, that they are most evident in Ammoccetes, and
become less and less marked in the higher vertebrates, it is clear
that their origin cannot be sought among the animals higher in the
scale than Ammocoetes, but must, therefore, be searched for in the
opposite direction.

Miiller describes them as forming a very conspicuous landmark in
the embryology of the retina, dividing it distinctly into two parts, an
outer thinner, and an inner somewhat thicker part, the zone formed
by them standing out conspicuously on account of the size and regu-
larity of the cells and their lighter appearance when stained. Thus
in his description of the retina of an Ammoccetes 95 mm. in length,
he says, " The layer of pale tangentially elongated cells formed a
double layer and produced the appearance of a pale, very charac-
teristic zone between the outer and inner parts of the retina."

Let us now turn to the retina of the crustacean and see whether
there is any evidence there that the retina is divisible into an outer
and inner part, separated by a zone of characteristically pale staining
cells with conspicuous nuclei. The most elaborate description of
the development of the retina of Astacus is given by Eeichenbach,

THE EVIDENCE OF THE ORGANS OF VISION 99

according to whom the earliest sign of the formation of the retina is an
ectodermic involution (Augen-einstulpung), which soon closes, so that
the retinal area appears as a thickening. In close contiguity to this
thickening, the thickening of the optic ganglion arises, so that that
part of the optic ganglion which will form the retinal ganglion fuses
with the thickened optic plate and forms a single mass of tissue.
Later on a fold (Augen-falte) appears in this mass of tissue, in conse-
quence of which it becomes divided into two parts. The lining walls
of this fold form a double row of cells, the nuclei of which are most
conspicuous because they are larger and lighter in colour than the
surrounding nuclei, so that by this fold the retina is divided into an
outer and an inner wall, the line of demarcation being conspicuous by
reason of these two rows of large, lightly-staining nuclei.

Eeichenbach is unable to say that this secondary fold is coincident
with the primary involution, and that therefore the junction between
the two rows of large pale nuclei is the line of junction between the
retinal ganglion and the retina proper, because all sign of the primary
involution is lost before the secondary fold appears.

Parker compares the appearances in the lobster with Reichenbach's
description in the crayfish, and says that he finds only a thicken-
ing, no primary involution ; at the same time he expressly states
that in the very early stages his material was deficient, and that he
had not grounds sufficient to warrant the statement that no involution
occurs. He also finds that in the lobster the ganglionic tissue which
arises by proliferation is divided into an outer and inner part ; the
separation is effected by a band of large, lightly-staining nuclei, which,
in position and structure, resemble the band figured by Eeichenbach.
According to Parker, then, the line of separation indicated in the
development by Reichenbach's outer and inner walls is not the line
of junction between the retina and the retinal ganglion, as Reichen-
bach was inclined to think, but rather a separation of two rows of
large ganglion-cells belonging to the retinal ganglion.

The similarity between these conspicuous layers of lightly-
staining cells in Ammoccetes and in crustaceans is remarkably close,
and in both cases observers have found the same difficulty in inter-
preting their meaning. In each case one group of observers looks
upon them as ganglion-cells, the other as supporting structures.
Thus in the lamprey, Muller considers them to belong to the support-
ing elements, while Langerhans and Kohl describe them as a double

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THE ORIGIN OF VERTEBRATES

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layer of ganglion-cells. In the crustacean, Berger in Squilla, Gren-

acher in Mysis, and Parker in Astacus, look upon them as supporting

elements, while Viallanes in
Palinurus considers them to be
true ganglionic cells.

Whatever the final interpre-
tation of these cells may prove
to be, we may, it seems to me,
represent an ideal compound
retina of the crustacean type by
combining the investigations of
Berger, Claus, Beichenbach, and
Parker in the following figure.

The comparison of this figure
(Fig. 42) with that of the Pe-
tromyzon retina (Pig. 41) shows
how great is the similarity of
the latter with the arthropod
type, and how the very points
in which it deviates from the
recognized vertebrate type are
explainable by comparison with
that of the arthropod. The
most striking difference between
the retinas in the two figures is
that the layer of terminal nerve
fibres (5, Fig. 42), which, after
all, are only the elongated termi-
nations of the retinal cells be-
longing to Parker's neurones of
the first order, is very much
longer than in Petromyzon or in
any vertebrate, for the external
molecular layer (G, Fig. 41)
(Muller's layer of Nervenan-

satze) is very short and inconspicuous (in Fig. 41 it is drawn too

thick).

Turning from the retina to the fibres of the optic nerve we again

find a remarkable resemblance, for in Ammoccetes, as pointed out by

Fig. 42. — Ideal Diageam of the Layers
in a Crustacean Eye.

The retina is divided into an epithelial
part, C (the layer of retinular cells and
rhabdomes), and a neurodermal or cere-
bral part, which is formed of, A, the
ganglion of the optic nerve, and, B, the
ganglion of the retina. 1, optic nerve
fibres which cross at (their entrance into
the retina ; 2, int. molecular layer with
its two rows of cells ; 3, int. nuclear
layer ; 4, Reichenbach's double row of
large lightly-staining cells ; 5, layer of
terminal retinal fibres ; 6, ext. nuclear
layer ; 7, ext. limiting membrane ; 8,
layer of crystalline cones ; 9, cornea.

THE EVIDENCE OF THE ORGANS OF VISION IOI

Langerkaiis and carefully figured l>y Kohl, a crossing of the fibres of
the optic nerve occurs as the nerve leaves the retina, just as is so uni-
versally the case in all compound retinas. To this crossing Kohl has
given the name chiasma nervi optici, in distinction to the cerebral
chiasma, which he calls chiasma nervorum opticorum. Further, we
find that even this latter chiasma is well represented in the arthro-
pod brain ; thus Bellonci in Sphgeroma, Berger, Dietl, and Krieger in
Astacus, all describe a true optic chiasma, the only difference in
opinion being, whether the crossing of the optic nerves is complete or
not. Especially instructive are Bellonci's figures and description.
He describes the brain of Sphaeroma as composed of three segments
— a superior segment, the cerebrum proper, a middle segment,
and an inferior segment ; the optic fibres, as is seen in Fig. 39,
after crossing, pass direct into the middle segment, in the ganglia of
which they terminate. From this segment also arises the nerve to
the first antenna of that side — i.e. the olfactory nerve. The optic
part, then, of this middle segment is clearly the brain portion of the
optic ganglionic apparatus, and may be called the optic lobes, in
contradistinction to the peripheral part, which is usually called the
optic ganglion, and is composed of two ganglia, Op. g. I. and Op. g. II.,
as already mentioned. These optic lobes are therefore homologous
with the optic lobes of the vertebrate brain.

The resemblance throughout is so striking as to force one to the
conclusion that the retina of the vertebrate eye is a compound retina,
composed of a retiua and retinal ganglion of the type found in arthro-
pods. From this it follows that the development of the vertebrate
retina ought to show the formation of (1) an optic plate formed
from the peripheral epidermis and not from the brain ; (2) a part of
the brain closely attached to this optic plate forming the retinal
ganglion, which remains at the surface when the rest of the optic
ganglion withdraws ; (3) an optic nerve formed in consequence of
this withdrawal, as the connection between the retinal and cerebral
parts of the optic ganglion.

This appears to me exactly what the developmental process does
show according to Gotte's investigations. He asserts that the retina
arises from an optic plate, being the optical portion of his ' Sinnes-
platte.' At an early stage this is separated by a furrow (Furche)
from the general mass of epidermal cells which ultimately form the
brain. This separation then vanishes, and the retina and brain-mass

102 THE ORIGIN OF VERTEBRATES

become inextricably united into a mass of cells, which are still
situated at the surface. By the closure of the cephalic plate and the
withdrawal of the brain away from the surface, a retinal mass of cells
is left at the surface connected with tjhe tubular central nervous
system by the hollow optic diverticulum or primary optic vesicle.
If we regard only the retinal and nervous elements, and for the
moment pay no attention to the existence of the tube, Gotte's obser-
vation that the true retina has been formed from the optic plate
(Sinnes-platte) to which the retinal portion of the brain (retinal
ganglion) has become firmly fixed, and that then the optic nerve has
been formed by the withdrawal of the rest of the brain (optic lobes),
is word for word applicable to the description of the development of
the compound retina of the arthropod eye, as has been already stated.

The Significance of the Optic Diverticula.

The origin of the retina from an optic epidermal plate in verte-
brates, as in all other animals, brings the cephalic eyes of all animals
into the same category, and leaves the vertebrate eye no longer in an
isolated and unnatural position. In one point the retina of the verte-
brate eye differs from that of a compound retina of an invertebrate ;
in the former, a striking supporting tissue exists, known as Midler's
fibres, which is absent in the latter. This difference of structure is
closely associated with another of the same character as in the central
nervous system, viz. the apparent development of the nervous part from
a tube. We see, in fact, that the retinal and nervous arrangements of
the vertebrate eye are comparable with those of the arthropod eye, in
precisely the same way and to the same extent as the nervous matter
of the brain of the vertebrate is comparable with the brain of the
arthropod. In both cases the nervous matter is, in structure, position,
and function, absolutely homologous ; in both cases there is found in
the vertebrate something extra which is not found in the invertebrate
— viz. a hollow tube, the walls of which, in the case of the brain, are
utilized as supporting tissues for the nerve structures. The explana-
tion of this difference in the case of the brain is the fundamental
idea of my whole theory, namely, that the hollow tube is in reality
the cephalic stomach of the invertebrate, around which the nervous
brain matter was originally grouped in precisely the same manner as
in the invertebrate. What, then, are the optic diverticula ?

THE EVIDENCE OF THE ORGANS OF VISION 103

" The formation of the eye," as taught by Balfour, " commences
with the appearance of a pair of hollow outgrowths from the anterior
cerebral vesicle. These outgrowths, known as the optic vesicles, at
first open freely into the cavity of the anterior cerebral vesicle.
From this they soon, however, become partially constricted, and
form vesicles united to the base of the brain by comparatively
narrow, hollow stalks, the rudiments of the optic nerves."

" After the establishment of the optic nerves, there takes place
(1) the formation of the lens, and (2) the formation of the optic cup
from the walis of the primary optic vesicle."

He then goes on to explain how the formation of the lens forms
the optic cup with its double walls from the primary optic vesicle,
and says —

" Of its double walls, the inner, or anterior, is formed from the
front portion, the outer, or posterior, from the hind portion of the
wall of the primary optic vesicle. The inner, or anterior, which very
speedily becomes thicker than the other, is converted into the retina ;
in the outer, or posterior, which remains thin, pigment is eventually
deposited, and it ultimately becomes the tesselated pigment-layer of
the choroid."

The difficulties in connection with this view of the origin of the
eye are exceedingly great, so great as to have caused Balfour to
discuss seriously Lankester's suggestion that the eye must have been
at one time within the brain, and that the ancestor of the vertebrate
was therefore a transparent animal, so that light might get to the eye
through the outer covering and the brain-mass ; a suggestion, the
unsatisfactory nature of which Balfour himself confessed. Is there
really evidence of any part of either retina or optic nerve being
formed from the epithelial lining of the tube ?

This tube is formed as a direct continuation of the tube of the
central nervous system, and we can therefore apply to it the same
arguments as have been used in the discussion of the meaning of the
latter tube. Now, the striking point in the latter case is the fact
that the lining membrane of the central canal is in so many parts
absolutely free from nervous matter, and so shows, as in the so-called
choroid plexuses, its simple, non-nervous epithelial structure. This
also we find in the optic diverticulum. Where there is no evidence
of any invasion of the tube by nervous elements, there it retains its
simple non-nervous character of a tube composed of a single layer of

104 THE ORIGIN OF VERTEBRATES

epithelial cells — viz. in that part of the tube which, as Balfour says,
remains thin, in which pigment is eventually deposited, and which
ultimately becomes the tesselated pigment-layer of the choroid.
Nobody has ever suggested that this pigment-layer is nervous matter,
or ever was, or ever will be, nervous matter ; it is in precisely the
same category as the membranous roof of the brain in Ammocoetes,
which never was, and never will be, nervous matter. Yet, according
to the old embryology both in the case of the eye and the brain, the
pigment-layer and the so-called choroid plexuses are a part of the
tubular nervous system.

Turning now to the optic nerve, Balfour describes it as derived
from the hollow stalk of the optic vesicle. He says —

" At first the optic nerve is ecrually continuous with both walls
of the optic cup, as must of necessity be the case, since the interval
which primarily exists between the two walls is continuous with the
cavity of the stalk. When the cavity within the optic nerve
vanishes, and the fibres of the optic nerve appear, all connection is
ruptured between the outer wall of the optic cup and the optic
nerve, and the optic nerve simply perforates the outer wall, and
becomes continuous with the inner one."

In this description Balfour, because he derived the optic nerve
fibres from the epithelial wall of the optic stalk, of necessity supposed
that such fibres originally supplied both the outer and inner walls of
the optic cup and, therefore, seeing that when the fibres of the optic
nerve appear they do not supply the outer wall, he supposes that
their original connection with the outer wall is ruptured, because a
discontinuity of the epithelial lining takes place coincidently witli
the appearance of the optic nerve-fibres, and, according to him, the
optic nerve simply perforates the outer wall and becomes continuous
with the inner one. This last statement is very difficult to under-
stand. I presume he meant that some of the fibres of the optic nerve
supplied from the beginning the inner wall of the optic cup, but
that others which originally supplied the outer wall were first ruptured,
then perforated the outer wall, and finally completed the supply to
the inner wall or retina.

This statement of Balfour's is the necessary consequence of his
belief, that the epithelial cells of the optic stalk gave rise to the
fibres of the optic nerve. If, instead of this, we follow Kolliker and
His, who state that the optic nerve-fibres are formed outside the

THE EVIDENCE OF THE ORGANS OF VISION 105

epithelial walls of the optic stalk, and that the cells of the latter
form supporting structures for the nerve-fibres, then the position of
the optic nerve becomes perfectly simple and satisfactory without
any rupturing of its connection with the outer wall and subsequent
perforation, for the optic nerve-fibres from their very first appearance
pass directly to supply the retina — i.e. the inner wall of the optic
cup and nothing else.

They pass, as is well known, without any perforation by way of
the choroidal slit to the inner surface of the inner wall (retina) of
the optic cup; then, when the choroidal
slit becomes closed by the expansion ?

of the optic cup, the optic nerve
naturally becomes situated in the centre
of the base of the cup and spreads over
its inner surface as that surface expands.

A section across the optic cup at an
early stage at the junction of the optic
stalk and optic cup would be repre-
sented by the upper diagram in Fig.
43 ; at a later stage, when the choroidal
slit is closed, by the lower diagram.

The evident truth of this manner
of looking at the origin of the optic
nerve is demonstrated by the appear-
ance of the optic nerve in Amrao-
ccetes and Fetromyzon. In the latter,
although the development is complete,
and the eye, and consequently also the
optic nerve-fibres, are fully functional,
there is still present in the axial core
of the nerve a row of epithelial cells
(Axenstrang) which are altered so as
to form supporting structures, in the
same way as a row of epithelial cells in the retina is altered to form
the system of supporting cells known by the name of the Mtillerian
fibres.

The origin of this axial core of cells is perfectly clear, as has been
pointed out by W. Miiller. He says—

" The development of the optic nerve shows peculiarities in

Fig. 43. — Diagram op the RELA-
tion of the optic nerve to
the Optic Cup.

The upper diagram represents a
stage before the formation of the
choroidal slit, the lower one the
stage of closure of the choroidal
slit. R., retina; O.n., optic
nerve ; p., pigment epithelium.

106 THE ORIGIN OF VERTEBRATES

Petromyzon of such a character as to make this animal one of the
most valuable objects for deciding the various controversial questions
connected with the genesis of its elements. The lumen of the stalk
of the primary optic vesicle is obliterated quite early by a prolife-
ration of its lining epithelium. Also the original continuity of this
epithelium with that of the pigment-layer is at an early period
interrupted at the point of attachment of the optic stalk. This
interruption occurs at the time when the fibres of the optic nerve
first become visible."

Further on he says —

" The epithelium of the optic stalk develops entirely into sup-
porting cells, which in Petromyzon fill up the original lumen and so
form an axial core (Axenstrang) to the nerve-fibres which are formed
entirely outside them ; the projections of these supporting cells are
directed towards the periphery, and so separate the bundles of the
optic nerve- fibres. The mesodermal coat of the optic stalk takes no
part in this separation ; it simply forms the connective tissue sheath
of the optic nerve. The development of the optic nerve in the
higher vertebrates also obeys the same law, as I am bound to conclude
from my own observations."

The evidence, then, of Ammococtes is very conclusive. Originally
a tube composed of a single layer of epithelial cells became expanded
at the anterior end to form a bulb. On the outside of this tube or
stalk the fibres of the optic nerve make their appearance, arising from
the ganglion-cell layer of the retina, and, passing over the surface of
the epithelial tube at the choroidal fissure, proceed to the brain by
way of the optic chiasma. Owing to the large number of fibres, their
crossing at the junction of the stalk with the bulb, and the narrow-
ness at this neck, the obliteration of the lumen of the tube which
takes place in the stalk is carried out to a still greater extent at this
narrow part. The result of this is that all continuity of the cell-
layers of the original tube of the optic stalk with those of both the
inner and outer walls of the bulb is interrupted, and all that remains
in this spot of the original continuous line of cells which connected the
tube of the stalk with that of the bulb are possibly some of the groups
of cells which are found scattered among the fibres of the optic nerve
at their entrance into the retina. Such separation of the originally
continuous elements of the epithelial wall of the optic stalk, which
is apparent only at this neck of the nerve in Petromyzon, takes place

THE EVIDENCE OF THE ORGANS OF VISION 107

along the whole of the optic nerve in the higher vertebrates, so that
no continuous axial core of cells exist, but only scattered supporting
cells.

If further proof in support of this view be wanted, it is given by
the evidence of physiology, which shows that the fibres of the optic
nerve are not different from other nerve-fibres of the central nervous
system, but that they degenerate when separated from their nerve-
cell, and that the nerve-cell of which the optic nerve-fibre is a
process is the large ganglion-cell of the ganglionic layer of the retina.
The origin of the ganglionic layer of the retina cannot therefore be
separated from that of the optic nerve-fibres. If the one is outside
the epithelial tube, so is the other, and what holds true of the gan-
glionic layer must hold good of the rest of the retinal ganglion and,
from all that has been said, of the retina itself. We therefore come
to the conclusion that the evidence is distinctly in favour of the
view, that the retina and optic nerve in the true sense are structures
which originally were outside a non-nervous tube, but, just like the
central nervous system as a whole, have amalgamated so closely with
the elements of this tube as to utilize them for supporting structures.
One part of this non-nervous tube, its dorsal wall, like the corre-
sponding part of the brain-tube, still retains its original character,
and by the deposition of pigment has been pressed into the service
of the eye to form the pigmented epithelial layer;

We can, however, go further than this, for we know definitely in
the case of the retina what the fate of the epithelial cells lining
this tube has been. They have become the system of supporting
structures known as Miillerian fibres.

The epithelial layer of the primary optic vesicle can be traced into
direct continuity with the lining epithelium of the brain cavity, as
a single layer of epithelial cells in the core of the optic nerve, form-
ing the optic stalk, which, in consequence of close contact, becomes
the well-known axial layer of supporting cells. This epithelial layer
of the optic stalk then expands to form the optic bulb, the outer or
dorsal wall of which still remains as a single layer of epithelium
and becomes the layer of pigment epithelium. This layer of
epithelium becomes doubled on itself by the approximation of the
inner or ventral wall of the optic cup to the outer or dorsal wall in
consequence of the presence of the lens, and still remaining a single
layer, forms the pars ciliaris retinae ; then suddenly, at the ora

io8

THE ORIGIN OF VERTEBRATES

serrata, the single epithelial layer vanishes, and the layers of the
retina take its place. It has long been known, however, that even
throughout the retina this single epithelial layer still continues, being
known as the fibres of Miiller. This is how the fact is described
in the last edition of Foster's "Text-book of Physiology," p. 1308 —

" Stretching radially from the inner to the outer limiting mem-
brane in all regions of the retina are certain peculiar- shaped bodies
known as the radial fibres of Miiller. Each fibre is the outcome of
the changes undergone by what was at first a simple columnar

epithelial cell. The changes
are, in the main, that the
columnar form is elongated
into that of a more or less
prismatic fibre, the edges of
which become variously
branched, and that while the
nucleus is retained the cell
substance becomes converted
into neuro-keratin. And, in-
deed, at the ora serrata the
fibres of Miiller may be seen
suddenly to lose their peculiar
features and to pass into the
ordinary columnar cells which
form the pars ciliaris retime."
It is then absolutely clear
that the essential parts of the
eye may be considered as
composed of two parts —

. p.c r

- P

- aa.t

Fig. 44. — Diagram representing the
Single-layered Epithelial Tube of
the Vertebrate Eye after removal of
the Nervous and Retinal Elements.

O.n., axial core of cells in optic nerve; 2}->
pigment epithelium; p.c.r., pars ciliaris
retina ; m.f., Miillerian fibres; I., lens.

1. A tube or diverticulum
from the tube of the central nervous system, composed throughout
of a single layer of epithelium, which forms the supporting axial
cells in the optic nerve, the pigment epithelium and the Miillerian
fibres of the retina. Such a tube would be represented by the
accompanying Fig. 44, and the left side of Fig. 41.

2. The retina proper with the retinal ganglion and the optic
nerve-fibres as already described. In this part supporting elements
are found, just as in any other compound retina, of the nature of
neuroglia, which are independent of the Miillerian fibres.

THE EVIDENCE OF THE ORGANS OF VIS I OX 1 09

Of these two parts we have already seen that the second is to
all intents and purposes a compound retina of a crustacean eye, and
seeing that the single-layered epithelial tube is continuous with the
single-layered epithelial tube of the central nervous system — i.e. with
the cephalic part of the gut of the arthropod ancestor — it follows with
certainty that the ancestor of the vertebrates must have possessed
two anterior diverticula of the gut, with the wall of which, near the
anterior extremity, the compound retina has amalgamated on either
side, just as the infra-cesophageal ganglia have amalgamated with
the ventral wall of the main gut-tube. In this way, and in this way
alone, does the interpretation of the structure of the vertebrate lateral
eye harmonize in the most perfect manner with the rest of the con-
clusions already arrived at.

The question therefore arises : — Have we any grounds for believing
that the ancient forms of primitive crustaceans and primitive arachnids,
which were so abundant in the time when the Cephalaspids appeared,
possessed two anterior diverticula of the stomach, such as the con-
sideration of the vertebrate eye strongly indicates must have been
the case ?

The beautiful pictures of Blanchard, and his description, show
how, on the arachnid side, paired diverticula of the stomach are
nearly universal in the group. Thus, although they are not present
in the scorpions, still, in the Thelyphonidae, Phrynidas, Solpugidae,
Mygalidse, the most marked characteristic of the stomach-region is
the presence of four pairs of ccecal diverticula, which spread laterally
over the prosomatic region. In the spiders the number of such
diverticula increases, and the whole prosomatic region becomes rilled
up with these tubes. Blanchard considers that they form nutrient
tubes for the direct nutrition of the organs in the prosoma, especially
the important brain-region of the central nervous system. He points
out that these animals are blood-suckers, and that, therefore, their
food is already in a suitable form for purposes of nutrition when it
is taken in by them, so that, as it were, the anterior part of the gut
is transformed into a series of vessels or diverticula conveying blood
directly to the important organs in the prosoma, by means of which
they obtain nourishment in addition to their own blood-supply.

The universality of such diverticula among the arachnids makes
it highly probable that their progenitors did possess an alimentary
canal with one or more pairs of anterior diverticula. In the

I IO

THE ORIGIN OF VERTEBRATES

vertebrate, however, the paired diverticula are associated with a
compound retina, a combination which does not occur among living
arachnids ; we must, therefore, examine the crustacean group for the
desired combination, and naturally the most likely group to examine
is the Phyllopoda, especially such primitive forms as Branchipus and
Artemia, for it is universally acknowledged that these forms are

.— rt.gl

Fig. 45. — Section through one of the two Anterior Diverticula of the Gut
in Artemia and the Retinal Ganglion.

The section is through the extreme anterior end of the diverticulum, thus cutting
through many of the columnar cells at right angles to their axis. AL, gut
diverticulum ; rt. gl., retinal ganglion.

the nearest living representatives of the trilobites. If, therefore, it
be found that the retina and optic nerve in Artemia is in specially
close connection with an anterior diverticulum of the gut on each
side, then it is almost certain that such a combination existed also
in the trilobites.

My friend Mr. W. B. Hardy has especially investigated the
nervous system of Artemia. In the course of his work he cut serial

THE EVIDENCE OF THE ORGANS OF VISION III

sections through the whole animal, and, as mentioned in my paper
in the Journal of Anatomy and Physiology, he discovered that the
retinal ganglion of each c.e On

lateral eye is so closely
attached to the end of the
corresponding diverticu-
lum of the gut that the
lining cells of the ventral
part of the diverticulum
form a lining to the reti-
nal ganglion (Fig. 45). In
this animal there are only
two gut-diverticula, which
are situated most ante-
riorly. I have plotted
out this series of sections
by means of a camera
lucida, with the result
that the retina appears as
a bulging attached ventro-
laterally to the extremity of each gut-diverticulum, as is shown in

Fig. 46. — The Brain, Eyes, and Anterior
Termination of the Alimentary Canal of
Artemia, viewed from the Dorsal Aspect.

Br., brain; I.e., lateral eyes; c.e., median eyes; Al.,
alimentary canal.

A B

Fig. 47.— A, The Formation of the Retina of the Eye of Ammoccetes (after
Scott) ;' B, The Formation of the Retina of the Eye of Ammoccetes, on

MY THEORY.

R., retina; l, lens; O.n., optic nerve fibres; Al., cephalic end of invertebrate ali-
mentary canal; V., cavity of ventricles of brain; Aid,, anterior diverticulum
of alimentary canal ; op.d., optic diverticulum.

Fig. 46. It is instructive to compare with this figure Scott's picture
of the developing eye in Ammoccetes, where he figures the retina as

112 THE ORIGIN OF VERTEBRATES

a bulging attached ventrally to the extremity of the narrow tube of
the optic diverticulum. In Fig. 47, A, I reproduce this figure of
Scott, and by the side of it, Fig. 47, B, I have represented the origin
of the vertebrate eye as 1 believe it to have occurred.

We see, then, this very striking fact, that in the most primitive
of the Crustacea, not only are there two anterior diverticula of the
gut, but also the retinal ganglion of the lateral eye is in specially
close connection with the end of the diverticulum on each side. In
fact, we find in the nearest living representative of the trilobites a
retina and retinal ganglion and optic nerve, closely resembling that
of the vertebrate, in close connection with an epithelial tube which
has nothing to do with the organ of sight, but is one of a pair of
anterior gut-diverticula. It is impossible to obtain more decisive
evidence that the trilobites possessed a pair of gut-diverticula sur-
rounded to a greater or less extent by the retina and optic nerve of
each lateral eye.

Such anterior diverticula are commonly found in the lower
Crustacea ; they are usually known by the name of liver-diverticula,
but as they take no part in digestion, and, on the contrary, represent
that part of the gut which is most active in absorption, the term
liver is not appropriate, and it is therefore better to call them simply
the pair of anterior diverticula. Our knowledge of their function in
Daphnia is given in a paper by Hardy and M'Dougall, which does
not appear to be widely known. Hardy succeeded in feeding Daphnia
with yolk of egg in which carmine grains were mixed, and was able
in the living animal to watch the whole process of deglutition,
digestion, and absorption. The food, which is made into a bolus, is
moved down to the middle region of the gut, and there digestion
takes place. Then by an antiperistaltic movement the more fluid
products of the digestion-process are sent right forward into the two
anterior diverticula, where the single layer of columnar cells lining
these diverticula absorbs these products, the cells becoming thickly
studded with fat-drops after a feed of yolk of egg. The carmine
particles, which were driven forward with the proteid- and fat-
particles, are not absorbed, but are at intervals driven back by con-
tractions of the anterior diverticula to the middle region of the gut.

These observations prove most clearly that the anterior diver-
ticula have a special nutrient function, being the main channels by
which new nutrient material is brought into the body, and, as

THE EVIDENCE OF THE ORGANS OF VISION 113

pointed out by the authors, it is a remarkable exception in the
animal kingdom that absorption should occur in that portion of the
gut which is anterior to the part in which digestion occurs. In all
these animals the two anterior diverticula extend forwards over the
brain, and, as we have seen in Artemia, the anterior extremity
of each one is so intimately related to a part of the brain — viz.
the retinal ganglion — as to form a lining membrane to that mass
of nerve-cells. It follows, therefore, that the nutrient fluid absorbed
by the cells of this part of the gut-diverticulum must be primarily
for the service of the retinal ganglion. In fact, the relations of
this anterior portion of the gut to the brain as a whole suggest
strongly that the marked absorptive function of this anterior
portion of the gut exists in order to supply nutrient material
in the first place to the most vital, most important organ in the
animal — the brain and its sense-organs. This conclusion is borne
out by the fact that in these lower crustaceans the circulation of
blood is of a very inefficient character, so that the tissues are mainly
dependent for their nutrition on the fluid immediately surrounding
them. It stands to reason that the establishment of the anterior
portion of the gut as a nutrient tube to the brain would necessitate
a closer and closer application of the brain to that tube, so that the
process of amalgamation of the brain with the single layer of columnar
epithelial cells which constitutes the wall of the gut (which we see
in its initial stage in the retinal ganglion of Artemia), would tend
rapidly to increase as more and more demands were made upon the
brain, until at last both the supra- and infra-cesophageal ganglia, as
well as the retinal ganglia and optic nerves, were in such close
intimate connection with the ventral wall of the anterior portion of
the gut and its diverticula as to form a brain and retina closely
resembling that of Ammoccetes.

Such an origin for the lateral eyes of the vertebrate explains in a
simple and satisfactory manner why the vertebrate retina is a com-
pound retina, and why both retina and optic nerve have an apparent
tubular development.

At the same time one discrepancy still exists which requires
consideration — viz. in no arthropod eye possessing a compound
retina is the retina inverted. All the known cases of inversion
among arthropods occur in eyes, the retina of which is simple, and
are all natural consequences of the process of invagination by which

114 THE ORIGIN OF VERTEBRATES

the retina is formed. On the other hand, eyes with an inverted
compound retina are not entirely unknown among invertebrates, for
the eyes of Pecten and of Spondylus possess a retina which is
inverted after the vertebrate fashion and still may be spoken of as
compound rather than simple. It is clear that an invagination, the
effect of which is an inversion of the retinal layer, would lead to
the same result, whether the retinal optic nerves were short or long,
whether, in fact, a retinal ganglion existed or not. Undoubtedly the
presence of the retinal ganglion tends greatly to obscure any process
of invagination, so that, as already mentioned, many observers, with
Parker, consider the retina of the crustacean lateral eye to be
formed by a thickening only, without any invagination, while
Peichenbach says an obscure invagination does take place at a very
early stage. So in the vertebrate eye most observers speak only of
a thickening to form the retina, but Gotte's observation points to an
invagination of the optic plate at an early stage. So also in the eye
of Pecten, Korschelt and Heider consider that the thickening, by
which the retina is formed according to Patten, in reality hides an
invagination process by means of which, as Biitschli suggests, an
optic vesicle is formed in the usual manner. The retina is
formed from the anterior wall of this vesicle, and is therefore
inverted.

The origin of the inverted retina of the vertebrate eye does not
seem to me to present any great difficulty, especially when one
takes into consideration the fact that the retina is inverted in the
arachnid group, only in the lateral eyes. The inversion is
usually regarded as associated with the tubular formation of the
vertebrate retina, and it is possible to suppose that the retina became
inverted in consequence of the involvement of the eye with the gut-
diverticulum. I do not myself think any such explanation is at all
probable, because I cannot conceive such a process taking place with-
out a temporary derangement — to say the least of it— of the power of
vision, and as I do not believe that evolution was brought about by
sudden, startling changes, but by gradual, orderly adaptations, and
as I also believe in the paramount importance of the organs of
vision for the evolution of all the higher types of the animal kingdom,
I must believe that in the evolution from the Arthropod to the
Gephalaspid, the lateral eyes remained throughout functional. I
therefore, for my own part, would say that the inversion of the

THE EVIDENCE OF THE ORGANS OF VISION I 15

retina took place before the complete amalgamation with the gut-
diverticulum, that, in fact, among the proto-crustacean, proto-
arachnid forms there were some sufficiently arachnid to have an
inverted retina, and at the same time sufficiently crustacean to
possess a compound retina, and therefore a compound inverted
retina after the vertebrate fashion existed in combination with the
anterior gut-diverticula. Thus, when the eye and optic nerve sank
into and amalgamated with the gut-diverticulum, neither the dioptric
apparatus nor the nervous arrangements would suffer any alteration,
and the animal throughout the whole process would possess organs
of vision as good as before or after the period of transition.

Further, not only the retina but also the dioptric apparatus of
the vertebrate eye point to its origin from a type that combined
the peculiarities of the arachnids and the crustaceans. In the
former it is difficult to speak of a true lens, the function of a lens
being undertaken by the cuticular surface of the cells of the corneagen
(Mark's ' lentigtn '), while in the latter, in addition to the corneal
covering, a true lens exists in the shape of the crystalline cones.
Further, these crustacean lenses are true lenses in the vertebrate
sense, in that they are formed by modified hypodermal cells, and
not bulgings of the cuticle, as in the arachnid. We see, in fact, that
in the compound crustacean eye an extra layer of hypodermal cells has
become inserted between the cornea and the retina to form a lens.
So also in the vertebrate eye the lens is formed by an extra layer of
the epidermal cells between the cornea and the retina. The fact that
the vertebrate eye possesses a single lens, though its retina is composed
of a number of ommatidia, while the crustacean eye possesses a lens
to each ommatidium, may well be a consequence of the inversion of
the vertebrate retina. It is most probable, as Korschelt and Heider
have pointed out, that the retina of the arachnid eyes is composed
of a number of ommatidia, just as in the crustacean eyes and
in the inverted eyes it is probable that the image is focussed on
to the pigmented tapetal layer, and thence reflected on to the
percipient visual rods. In such a method of vision a single lens is a
necessity, and so it must also be if, as I suppose, eyes existed with
an inverted compound retina. Owing to the crustacean affinities of
such eyes, a lens would be formed and the retina would be compound :
owing to the arachnid affinities, the retina would be inverted and
the hypodermal cells which formed the lens would be massed

Il6 THE ORIGIN OF VERTEBRATES

together to form a single lens, instead of being collected in groups of
four to form a series of crystalline cones.

To sum up : The study of the vertebrate eyes, both median and
lateral, leads to most important conclusions as to the origin of the
vertebrates, for it shows clearly that whereas, as pointed out in this
and subsequent chapters, their ancestors possessed distinct arachnid
characteristics, yet that they cannot have been specialized arachnids,
such as our present-day forms, but rather they were of a primitive
arachnid type, with distinct crustacean characteristics : animals
that were both crustacean and arachnid, but not yet specialized in
either direction : animals, in fact, of precisely the kind which
swarmed in the seas at the time when the vertebrates first made their
appearance. In the opinion of the present day, the ancestral forms
of the Crustacea, which were directly derived from the Annelida,
may be classed as an hypothetical group the Protostraca, the nearest
approach to which is a primitive Phyllopod.

" Starting from the Protostraca," say Korschelt and Heider,
" according to the present condition of our knowledge, we may, as
has been already remarked, assume three great series of development
of the Arthropodan stock, by the side of which a number of smaller
independent branches have been retained. One of these series leads
through the hypothetical primitive Phyllopod to the Crustacea ; the
second through the Pakeostraca (Trilobita, Gigantostraca, Xiphosura)
to the Arachnida ; the third through forms resembling Peripatus to
the Myriapoda and the Insecta. The Pantapoda and the Tardigrada
must probably be regarded as smaller independent branches of the
Arthropodan stock."

To these " three great series of development of the Arthropodan
stock " the evidence of Ammocoetes shows that a fourth must be added,
which, starting also from the Protostraca, and closely connected with
the second, palffiostracan branch, leads through the Cephalaspidae to
the great kingdom of the Vertebrata. Such a direct linking of the
earliest vertebrates with the Annelida through the Protostraca is of
the utmost importance, as will be shown later in the explanation of
the origin of the vertebrate ccelom and urinary apparatus.

THE EVIDENCE OF THE ORGANS OF VISION 1 1 7

Summary.

The most important discovery of recent years which gives a direct clue to
the ancestry of the vertebrates is undoubtedly the discovery that the pineal gland
is all that remains of a pair of median eyes which must have been functional in
the immediate ancestor of the vei'tebrate, seeing1 how perfect one of them
still is in Ammocoetes. The vertebrate ancestor, then, possessed two pairs of
eyes, one pair situated laterally, the other median. In striking confirmation of
the origin of the vertebrate from Palaeostracans it is universally admitted that
all the Eurypterids and such-like forms resembled Limulus in the possession of
a pair of median eyes, as well as of a pair of lateral eyes. Moreover, the ancient
mailed fishes the Ostracodermata, which are the earliest fishes known, are all said
to show the presence of a pair of median eyes as well as of a pair of lateral eyes.
This evidence 'directly suggests that the structure of both the median and
lateral vertebrate eyes ought to be very similar to that of the median and lateral
arthropod eyes. Such is, indeed, found to be the case.

The retina of the simplest form of eye is formed from a group of the superficial
epidermal cells, and the rods or rhabdites are formed from the cuticular covering
of these cells ; the optic nerve passes from these cells to the deeper-lying brain.
This kind of retina may be called a simple retina, and characterizes the eyes,
both median and lateral, of the scorpion group.

In other cases a portion of the optic ganglion remains at the surface, when
the brain sinks inwards, in close contiguity to the epidermal sense-cells which
form the retina ; a tract of fibres connects this optic ganglion with the under-
lying brain, and is known as the optic nerve. Such a retina may be called
a compound retina and characterizes the lateral eyes of both crustaceans and
vertebrates. Also, owing to the method of formation of the retina by invagina-
tion, the cuticular surface of the retinal sense-cells, from which the rods are
formed, may be directed towards the source of light or away from it. In the
first case the retina may be called upright, in the second inverted.

Such inverted retinas are found in the vertebrate lateral eyes and in the
lateral eyes of the arachnids, but not of the crustaceans.

The evidence shows that all the invertebrate median eyes possess a simple
upright retina, and in structure are remarkably like the right median or pineal
eye of Ammocoetes ; while the lateral eyes possess, as in the crustaceans, an
upright compound retina, or, as in many of the arachnids, a simple inverted
retina. The lateral eyes of the vertebrates alone possess a compound inverted
retina.

This retina, however, is extraordinarily similar in its structure to the
compound crustacean retina, and these similarities are more accentuated in the
retina of the lateral eye of Petromyzon than that of the higher vertebrates.

The evidence afforded by the lateral eye of the vertebrate points unmistakably
to the conclusion that the ancestor of the vertebrate possessed both crustacean
and arachnid characters — belonged, therefore, to a group of animals which gave
rise to both the crustacean and arachnid groups. This is precisely the position
of the Palfeostracan group, which is regarded as the ancestor of both the
crustaceans and arachnids.

Il8 THE ORIGIN OF VERTEBRATES

In two respects the retina of the lateral eyes of vertebrates differs from that
of all arthropods, for it possesses a special supporting- structure, the Mullerian
fibres, which do not exist in the latter, and it is developed in connection with
a tube, the optic diverticulum, which is connected on each side with the main
tube of the central nervous system. These two differences are in reality one
and the same, for the Miillerian fibres are the altered lining cells of the optic
diverticulum, and this tube has the same significance as the rest of the tube of
the nervous system ; it is something which has nothing to do with the nervous
portion of the retina but has become closely amalgamated with it. The explana-
tion is. word for word, the same as for the tubular nervous system, and shows that
the ancestor of the vertebrate possessed two anterior diverticula of its alimentary
canal which were in close relationship to the optic ganglion and nerve of the
lateral eye on each side. It is again a striking coincidence to find that
Ai-temia, which with Branchipus represents a group of living crustaceans most
nearly allied to the trilobites, does possess two anterior diverticula of the gut
which are in extraordinarily close relationship with the optic ganglia of the
retina of the lateral eyes on each side.

The evidence of the optic apparatus of the vertebrate points most remarkably
to the derivation of the Vertebrata from the Palfeostraca.

CHAPTER III

THE EVIDENCE OF THE SKELETON

The bony and cartilaginous skeleton considered, not the notochord. — Nature of
the earliest cartilaginous skeleton. — The mesosomatic skeleton of Amnio-
ccetes ; its topographical arrangement, its structure, its origin in muco-
cartilage. — The prosomatic skeleton of Amnioccetes ; the trabecular and
parachordals, their structure, their origin in white fibrous tissue. — The
mesosomatic skeleton of Linmlus compared with that of Ammoccetes ;
similarity of position, of structure, of origin in muco-cartilage. — The
prosomatic skeleton of Linmlus ; the entosternite or plastron compared with
the trabecular of Ammocoetes; similarity of position, of structure, of origin
in fibrous tissue. — Summary.

The explanation of the two optic diverticula given in the last chapter
accounts in the same harmonious manner for every other part of the
tube around which the central nervous system of the vertebrate has
been grouped. The tube conforms in all respects to the simple epi-
thelial tube which formed the alimentary canal of the ancient type of
marine arthropods such as were dominant in the seas when the verte-
brates first appeared. The whole evidence so far is so uniform and
points so strongly in the direction of the origin of vertebrates from
these ancient arthropods, as to make it an imperative duty to proceed
further and to compare one by one the other parts of the central
nervous system, together with their outgoing nerves in the two groups
of animals.

Before proceeding to do this, it is advisable first to consider
the question of the origin of the vertebrate skeletal tissues, for this
is the second of the great difficulties in the way of deriving verte-
brates from arthropods, the one skeleton being an endo-skeleton
composed of cartilage and bone, and the other an exo-skeleton com-
posed of chitin. Here is a problem of a totally different kind to that
we have just been considering, but of so fundamental a character that
it must, if possible, be solved before passing on to the consideration
of the cranial nerves and the organs they supply.

120 THE ORIGIN OF VERTEBRATES

Is there any evidence which makes it possible to conceive the
method by which the vertebrate skeleton may have arisen from the
skeletal tissues of an arthropod ? By the vertebrate skeleton I mean
the bony and cartilaginous structures which form the backbone, the
cranio-facial skeleton, the pectoral and pelvic girdles, and the bones
of the limbs. I do not include the notochord in these skeletal tissues,
because there is not the slightest evidence that the notochord played
any part in the formation of these structures ; the notochordal tissue
is something mi generis, and never gives rise to cartilage or bone.
The notochord happens to lie in the middle line of the body and is
very conspicuous in the lowest vertebrate ; with the development of
the backbone the notochord becomes obliterated more and more, until
at last it is visible in the higher vertebrates only in the embryo ; but
that obliteration is the result of the encroachment of the growing
bone-masses, not the cause of their growth. Although, then, the
notochord may in a sense be spoken of as the original supporting axial
rod of the vertebrate, it is so different to the rest of the endo-skeleton,
has so little to do with it, that the consideration of its origin is a thing
apart, and must be treated by itself without reference to the origin of
the cartilaginous and bony skeleton.

The Commencement of the Bony Skeleton in the Vertebrate.

What is the teaching of the vertebrate ? What evidence is there
as to the origin of the bony skeleton in the vertebrate phylum
itself ?

The axial bony skeleton of the higher Mammalia consists of two
parts, (1) the vertebral column with its attached bony parts, and
(2) the cranio-facial skeleton. Of these two parts, the bony tissue
of the first arises in the embryo from cartilage, of the second partly
from cartilage, partly from membrane.

In strict accordance with their embryonic origin is their phyloge-
netic origin : as we pass from the higher vertebrates to the lower
these structures can be traced back to a cartilaginous and mem-
branous condition, so that, as Parker has shown, the cranio-facial
bony skeleton of the higher vertebrates can be derived directly from
a non-bony cartilaginous skeleton, such as is seen in Petromyzon
and the cartilaginous fishes.

Balfour, in his " Comparative Embryology," states that the

THE EVIDENCE OF THE SKELETON

121

primitive cartilaginous cranium is always composed of the following
parts : —

1. A pair of cartilaginous plates on each side of the cephalic
section of the notochord known as the parachordals (pa.ch., Fig. 49 ;
iv., Fig. 48). These plates, together with the notochord (ch.) enclosed
between them, form a floor for the hind and mid-braiu.

<S^--Ctr

-au

Fig. 48. — Embryo Pig, two-thirds of an
inch long (from Parker), Elements
of Skull seen from below.

ch., notochord; iv., parachordals; au.,
auditory capsule ; py., pituitary body ; tr.,
trabecula; ctr., trabecular cornu ; pn.,
pre-nasal cartilage ; ppg., palato-pterygoid
tract; mn., mandibular arch; th.h., first
branchial arch ; VII.-XIL, cranial nerves.

Fig. 49. — -Head of Embryo Dog-fish
(from Parker), Basal View of Cranium

FROM ABOVE.

ul., olfactory sacs; au., auditory capsule;
py., pituitary body; pa.ch., parachordal
cartilage; tr., trabecula; inf., infundi-
bulum ; pt.s., pituitary space ; c, eye.

2. A pair of bars forming the floor for the fore-brain, known as
the trabecular (tr). These bars are continued forward from the para-
chordals. They meet posteriorly and embrace the front end of the
notochord, and after separating for some distance bend in again in
such a way as to enclose a space — the pituitary space (pt.s.). In

122 THE ORIGIN OF VERTEBRATES

front of this space they remain in contact, and generally unite. They
extend forward into the nasal region (pn.).

3. The cartilaginous capsules of the sense organs. Of these the
auditory {an.) and the olfactory capsules (ol.) unite more or less inti-
mately with the cranial walls ; while the optic capsules, forming the
usually cartilaginous sclerotics, remain distinct.

The parachordals and notochord form together the basilar plate,
which forms the floor for that section of the brain belonging to
the primitive postoral part of the head, and its extent corresponds
roughly to that of the basioccipital of the adult skull.

The trabecular, so far as their mere anatomical relations are con-
cerned, play the same part in forming the floor for the front cerebral
vesicle as do the parachordals for the mid- and hind-brain. They
differ, however, from the parachordals in one important feature, viz.
that except at their hinder end they do not embrace the notochord.
The notochord always terminates at the infundibulum, and the
trabecular always enclose a pituitary space, in which lies the infun-
dibulum (inf.) and the pituitary body (py.).

In the majority of the lower forms the trabecular arise quite inde-
pendently of the parachordals, though the two sets of elements soon
unite.

The trabecular are usually somewhat lyre-shaped, meeting in
front and behind, and leaving a large pituitary space between their
middle parts. Into this space the whole base of the fore-brain
primitively projects, but the space itself gradually becomes narrowed
until it usually contains only the pituitary body.

The trabecular floor of the brain does not long remain simple.
Its sides grow vertically upwards, forming a lateral wall for the
brain, in which in the higher types, two regions maybe distinguished,
viz. an alisphenoidal region behind, growing out from what is known
as the basisphenoidal region of the primitive trabecular, and an
orbito-sphenoidal region in front, growing out from the presphenoidal
region of the trabecular. These plates form at first a continuous lateral
wall of the cranium. The cartilaginous wails which grow up from the
trabecular floor of the cranium generally extend upwards so as to form
a roof, though almost always an imperfect roof, for the cranial cavity.
The basi-cranial cartilaginous skeleton reduces itself always into
trabecular and parachordals with olfactory and auditory cartilaginous
capsules. ,

THE EVIDENCE OF THE SKELETON

123

An anterior arch known as the
C3 h-v*

In addition, a branchial skeleton exists, which consists of a series
of bars known as the branchial bars, so situated as to afford support
to the successive branchial pouches,
mandibular arch (Fig. 50,
Mn.), placed in front of the
hyo-rnandibular cleft, and
a second arch, known as the
hyoid arch (Hy.), placed in
front of the hyo-branchial
cleft, are developed in all
types; the succeeding arches
are known as the true bran-
chial arches (Br.), and are
only fully developed in the
Ichthyopsida. In all cases
of jaw-bearing (gnathosto-
matous) vertebrates the first
arch has become a support-
ing skeleton for the mouth (Fig. 51), and in the higher vertebrates in
combination with the second or hyoid arch takes part in the formation
of the ear-bones.

The true branchial arches persist, to a certain extent, in the

cr~

Mn Hu Bri
Hm Na Tr

Fig. 50. — Head op Embryo Dog-fish, eleven
lines long. (From Parker.)

Tr., trabecula ; Mn., mandibular cartilage ; Hy.,
hyoid arch; -Br,., first branchial arch; Na.,
olfactory sac ; E., eye ; An., auditory capsule ;
Hm., hemisphere; C,, C2, Cz, cerebral vesicles.

Ku'Htj

Fig. 51. — Skull op Adult Dog-pish, Side View. (From Parker.)
cr., cranium; Br., branchial arches; Mn. + Hy., mandibular and hyoid arches.

Amphibia, and become still more degenerated in the Amniota
(reptiles, birds, and mammals) in correlation with the total dis-
appearance of a branchial respiration at all periods of their life.

124 THE ORIGIN OF VERTEBRATES

Their remnants become more or less important parts of the hyoid
bone, and are employed solely in support of the tongue.

In no single animal is there any evidence that the foremost arch,
the mandibular, is a true branchial arch. As low down as the
Elasmobranchs it becomes divided into two elements which form
respectively the upper and lower jaws ; the hyoid arch, on the other
hand, although it has altered its form and acquired the secondary
function of supporting the mandibular arch, still retains its respi-
ratory function.

The evidence afforded by the mode of formation of the skeletal
tissues of vertebrates down to the Elasmobranchs indicates that the
primitive cranial skeleton arose from two paired basal cartilages, the
parachordals and trabecule, to which were attached respectively
cartilaginous cases enclosing the organs of hearing and smell. In
addition, the branchial portion of the cranial region was provided
with cartilaginous bars arranged serially for the support of the
branchiae, with the exception of the foremost, the mandibular bar,
which formed supporting tissues for the mouth — the upper and
lower jaws.

Just as in past times the spinal nerves and the segments they
supplied were supposed to represent the type on which the original
vertebrate was built, so also the spinal vertebrae afforded the type of
the segmented skeleton, and the anatomists of those days strove hard
to resolve the cranio-facial skeleton into a series of modified vertebrae.
Owing especially to the labours of Huxley, who showed that the seg-
mentation in the head-region was essentially a segmentation due to
the presence of branchial bars, this conception was finally laid to rest
and nowadays it is admitted to be hopeless to resolve the cranium
into vertebral segments. Still, however, the vertebrate is a segmented
animal and its segmented nature is visible in the cranial region, so far
as the skeletal tissues are concerned, in the shape of the series of
branchial and visceral bars.

To this segmentation the name of ' branchiomeric ' has been given,
while that due to the presence of vertebrae is called ' mesomeric'

As we have seen, the internal bony skeleton of the vertebrate
commences as a cartilaginous and membranous skeleton. For this
reason the preservation of such skeletons is impossible in the fossil
form, unless the cartilage has become impregnated with lime salts,
so that there is but little hope of ever obtaining traces of such

THE EVIDENCE OF THE SKELETON

125

structures in the fossils of the Silurian age either among the verte-
brate or invertebrate remains. Fortunately for this investigation
there are still living on the earth two representatives of that age ; on
the invertebrate side Limulus, and on the vertebrate side Ammoccetes.

The Elasmobranchs represent the most primitive of the gnatho-
stomatous vertebrates. Below them come the Agnatha, known as the
cyclostomatous fishes or Marsipobranchii, the lampreys (Petromyzon)
and the hag-fishes (Myxine).

The skeleton of Petromyzon (Fig. 52) consists of a cranio-facial
skeleton composed of a cartilaginous unsegmented cranium, with the
basal trabecule and parachordals and a series of branchial and visceral
cartilaginous bars forming the so-called branchial basket-work ; to
these must be added auditory and nasal capsules. In contradis-
tinction to this elaborate cranio-facial skeleton, the spinal vertebral

Fig. 52. — Skeleton of Petromyzox. (From Parker.)
na., nasal capsule; an., auditory capsule; nc, uotochord.

skeleton is represented only by segmen tally arranged small pieces of
cartilage formed in the connective tissue dissepiments between
segmented sheets of body-muscles (myotomes).

But Petromyzon is derived from Ammoccetes by a remarkable
process of transformation, and a most important part of that trans-
formation is the formation of new cartilaginous structures. Thus we
see that in Ammoccetes there is no sign of a cartilaginous vertebral
column ; at transformation the rudimentary vertebras of Petromyzon
are formed. In Ammoccetes the brain-case is a simple fibrous mem-
branous covering ; at transformation this becomes cartilaginous. In
Ammoccetes there are no cartilaginous structures corresponding to
the sub-ocular arches ; these are all formed at transformation. It
follows, that we can trace back the bony skeleton of the vertebrate
head to the skeleton of Ammoccetes, and we may therefore conclude

126

THE ORIGIN OF VERTEBRATES

that the primitive cartilaginous skeleton of the vertebrate consisted
of the following structures (Fig. 53, B), viz. the branchial bars

forming a basket-work, the

Ent

trabecule and parachordals,
the auditory and nasal cap-
sules— a clear proof that the
cranial skeleton is older than
the spinal. Of these struc-
tures the branchial bars are
the only evidently segmented
parts.

The Soft Cartilage of the
Branchial Skeleton of
Ammoccetes.

The study of Ammoccetes
gives yet another clue to the
nature of the earliest skeleton,
for these two marked groups
of cartilage — the branchial and
basi-cranial — are characterized
by a difference in structure as
well as a difference in topo-
graphical position. J. Miiller
was the first to point out that
these two sets of cartilages
differ in appearance and con-
stitution, and he gave to them"
the name of yellow and grey
cartilage. Parker has described
them fully under the terms
soft and hard cartilage, terms
which Schaffer has also used,
and I shall also make use of
them here. The whole of the
branchial cartilaginous skele-
ton is composed of soft cartilage, while the basi-cranial skeleton, con-
sisting of trabecule, parachordals, and auditory capsule, is composed

Fig. 53. — Comparison of Cartilaginous
Skeleton of Limulus and Ammoccetes.

A, Diagram of cartilaginons skeleton of
Limulus. Soft cartilage, entapophysial liga-
ments, deep black ; branchial bars simply
hatched; liard cartilage, lateral trabecule
of entosternite, netted ; Ph., position of
pharynx.

B, Diagram of cartilaginous skeleton of
Ammoccetes. Soft cartilage, sub-chordal
cartilaginous bands, deep black ; branchial
basket-work (first formed part), simply
hatched ; hard cartilage, cranio-facial skele-
ton, trabecule, parachordals and auditory
capsules, netted; Inf., position of tube of
infundibulum (old oesophagus).

THE EVIDENCE OF THE SKELETON 1 27

of hard cartilage, the only soft cartilage in this region being that
which forms the nasal capsule, not represented in Fig. 53, B.

These two groups of cartilage arise independently, so that at first
the basi-cranial system is quite separate from the branchial, and only
late in the history of the animal is a junction effected between the
branchial system and the trabecular and parachordals, an initial
separation which is especially striking when we consider that in this
animal all the cartilaginous structures of any one system are con-
tinuous : there is no sign of anything in the nature of joints.

Of these two main groups, the branchial cartilages are formed first
in the embryo, a fact which suggests that they are the most primi-
tive of the vertebrate cartilages, and that, therefore, the first true
formation of cartilage in the invertebrate ancestor may be looked for in
the shape of bars supporting the branchial mechanism. The evidence
of the origin of the cartilaginous structures in Ammoccetes is given
by Shipley in the following words : —

" The branchial bases are the first part of the skeleton to appear.
They arise about the 24th day as straight bars of cartilage, lying
external and slightly posterior to the branchial vessel.

" The first traces of the basi-cranial skeleton appear on the 30th
day as two rods of cartilage — the trabecular."

Our attention must, in the first place, be directed to this branchial
basket-work of Ammoccetes.

Underlying the skin of Ammoccetes in the branchial region is
situated the sheet of longitudinal body-muscles, divided into a series
of segments or myotomes, which forms the somatic muscles so cha-
racteristic of all fishes. This muscular sheet is depicted on the left-
hand side of Fig. 54. It does not extend over the lower lip or over
that part in the middle line where the thyroid gland is situated. In
these parts a sheet of peculiar tissue known by the name of muco-
cartilage lies immediately under the skin, covering over the thyroid
gland and lower lip. The somatic muscular sheet with the super-
jacent skin can be stripped off very easily owing to the vascularity
and looseness of the tissue immediately underlying it. When this is
done the branchial basket-work comes beautifully into view as is
seen on the right-hand side of Fig. 54. It forms a cage within which
the branchiar and their muscles lie entirely concealed.

This is the great characteristic of this most primitive form of the
branchial cartilaginous bars and distinguishes it from the branchial

128

THE ORIGIN OF VERTEBRATES

bars of other higher fishes, in that it forms a system of cartilages
which lie external to the branchia) — an extra-branchial system.

This branchial basket-work is simpler in Ammoccetes than in
Petromyzon, and its actual starting-point consists of a main trans-
verse bar corresponding to each branchial
segment ; from this transverse bar the
system of longitudinal bars by which
the basket-work is formed has sprung.
These transverse bars arise from a
cartilaginous longitudinal rod, situated
close against the notochord on each
side. These rods may be called the
subchordal cartilaginous bands (Fig. 53),
and, according to the observations .of
Schneider and others, each subchordal
band does not form at first a continuous
cartilaginous rod, but the cartilage is
conspicuous only at the places where
the transverse bars arise. In the
youngest Ammoccetes examined by
Schaffer, he could find no absolute dis-
continuity of the cartilage except be-
tween the first two transverse bars, but
he says that the thinning between the
transverse bars was so marked as to
make it highly probable that at an
earlier stage there was discontinuity.
The whole system of branchial bars and
subchordal rods is at first absolutely
disconnected from the cranial system of
trabecule and parachordals, and only
later do the two systems join.

These observations on Ammoccetes
lead most definitely to the conclusion
that the starting-point of the whole cartilaginous skeleton of
the vertebrate consisted of a series of transverse cartilaginous bars,
for the purpose of supporting branchial segments ; these were con-
nected with two axial longitudinal cartilaginous rods, which at first
contained cartilage only near the places of junction of the branchial

Fig. 54. — Ventral View of
Head Region of Ammoccetes.

Th., thyroid gland; M., lower
lip, with its muscles.

THE EVIDENCE OF THE SKELETON 1 29

bars. This system may be called the mesosomatic skeleton, as it is
entirely confined to the branchial or mesosomatic region.

In addition to this primitive cartilaginous framework, which was
formed for the support of the mesosomatic or respiratory segments,
but at a slightly -later period in the phylogenetic history, a separate
cartilaginous system was formed for the support of the prosomatic
segments, viz. the trabecular and parachordals with the auditory cap-
sules : a system which was at first entirely separated from the mesoso-
matic, and, as we shall see, is more advanced in structure than the
branchial system. Later still, the story is completed at the time of
transformation to Petromyzon by the formation of the simple cartila-
ginous skull and the rudimentary vertebrae, the structure of which
is also of a more advanced type.

The Structure of the Soft Branchial Cartilage.

Having considered the topographical position of the primitive
branchial cartilaginous skeleton, we may now inquire, What was
its structure and how was it formed ?

In the higher vertebrates various forms of cartilage are described,
viz. hyaline, fibro-cartilage, elastic cartilage, and parenchymatous
cartilage. Of these, the parenchymatous cartilage is looked upon as
the most primitive form, because it preserves without modification
the characters of embryonic cartilage.

Embryology, then, would lead to the belief that the earliest form
of cartilage in the vertebrate kingdom ought to be of this type, viz.
large cells, each of which is enclosed in a simple capsule, so that the
capsules of the cells form the whole of the matrix, and thus form a
simple homogeneous honeycomb-structure, in the alveoli of wrhich
the cartilage-cells lie singly. If, then, the branchial cartilages of
Ammocoetes are, as has just been argued, the representatives of the
cartilaginous skeleton of the primitive vertebrate, it is reasonable to
suppose that they should resemble in structure this embryonic car-
tilage. Such is undoubtedly the case : all observers who have
described the branchial basket-work of Ammocoetes or Petromyzon
have been struck with the extremely primitive character of the car-
tilage, and the last observer (Schafi'er) describes it as composed of
thin walls of homogeneous material, in which there are no lines of
separation, which form a simple honeycomb-structure, in the alveoli

13O THE ORIGIN OF VERTEBRATES

of which the separate cells lie singly. These branchial cartilages are
each surrounded by a layer of perichondrium, and in Fig. 55, A, I
give a picture of a section of a portion of one of the bars.

A B

Fig. 55. — A, Branchial Cartilage of Ammoccetes, stained with Thionin. B,
Branchial Cartilage of Limulus, stained with Thionin.

Hence we see that structurally as well as topographically the
branchial bars of Ammoccetes justify their claim to be considered as
the origin of the vertebrate cartilaginous framework.

On the Structure of the Muco-cartilage in Ammoccetes.

We can, however, go further than this, and ask how this cartilage
itself is formed in Ammoccetes ? The answer is most definite, most
instructive and suggestive, for in all cases this particular kind of car-
tilage is formed from, or at all events in, a peculiar fibrous tissue,
which was called by Schneider " Schleim-Knorpel,''' or muco-cartilage,
a tissue which is distinguishable from other connective tissues, not
only by its structural peculiarities, but also by its strong affinity for
all dyes which differentiate mucoid or chondro-mucoid substances.

This muco-cartilage is thus described by Schneider : — The peri-
chondrium in Ammoccetes is not confined to the true cartilaginous
structures, but extends itself in the form of thin plates in definite
directions. Between these plates of perichondrium a peculiar tissue
(Fig. 56) — the muco-cartilage — exists, consisting of fibrillar, whose
direction is mainly at right angles to the planes of the perichondrial
plates, with star-shaped cells in among them, and with the spaces
between the fibrillse filled up with a semi-fluid mass.

THE EVIDENCE OF THE SKELETON 131

From this tissue all the primitive cartilages which resemble the
branchial bars are formed, either by the invasion of chondroblasts
from the surrounding perichondrium, or by the proliferation and
encapsulation of the cells of the muco- cartilage itself.

This very distinctive tissue — the muco-cartilage — is of very great
importance in all questions of the origin of the skeletal tissues. In
all descriptions of the skeletal tissues it has been practically dis-
regarded until recent years when, besides my own observations, its
distribution has been mapped out by Schaffer. Thus Parker, in his
well-known description of the skeleton of the marsipobranch fishes,
does not even mention its existence. Its importance is shown by its
absolute disappearance at
transformation and its non-
occurrence in any of the

higher vertebrates. It is \PWcH i ' SvfH\J ■ 4 tot^'l

entirely confined to the head- ^ WU \, ) lli-r ','? f P^m m>-^

ton «■

region, and its distribution r'^'i w<*^f \P s':

there is most suggestive, for,

scr
later on, it forms a skele

Tmmwm

ton which both in structure ,' V 1 'IfjJnJ Vf

\ S iw» - n^.^ . ., (I ' .W— i ;

)

is will be described fully *>»?$:,*/
Later on, it forms a skele-
ton which both in structure
and position resembles very

closely the head-shields of ~W~JXZ-i*&3i~^^' '^"
cephalaspidian fishes. At '"- — ; C "N\ (

the present part of my argu- "~-~- — J v .

ment its more immediate Pig. 56.— Section of Muco-cartilage from
interest lies in the method Dorsal Head-plate of Ammoccetes.

of tracing this tissue. For

this purpose I made use of the micro-chemical reaction of thionin,
a dye which, as shown by Hoyer, stains all mucin-containing sub-
stances a bright purple. Schaffer made use of a corresponding
basophil stain, hsemalum. When stained with thionin, the matrix,
or ground-substance of the branchial cartilages as well as the matrix
or semi-fluid substance in which the fibrils of the muco-cartilaginous
cells are embedded take on a deep purple colour, while the fibrous
material of the cranial walls and other connective tissue strands, such
as the perichondrium, are coloured light blue. Muco-cartilage, then,
may be described as a peculiar form of connective tissue which
differs from other connective tissue not only in its appearance but in

132 THE ORIGIN OF VERTEBRATES

its chemical composition, for unlike white fibrous tissue it contains a
large amount of mucin, and this tissue is the forerunner of the earliest
cartilaginous vertebrate skeleton, the branchial bars of Amnioccetes.

The conclusions to which we are led by the study of the structure,
position, and mode of origin of these primitive cartilages of
Ammoccetes may be thus summed up : — -

1. The immediate ancestor of the vertebrate must have possessed
a peculiar fibrous tissue — the ground-substance of which stained deep
purple with thionin — in which cartilage arose.

2. The cartilage so formed was not like hyaline cartilage, but
resembled in a striking manner parenchymatous cartilage.

3. This cartilage was situated partly in two axial longitudinal
bands, partly as transverse bars, which supported the branchial
apparatus.

The Prosomatic or Basi-cranial Skeleton of Ammoccetes.

Before searching for any evidence of a similar tissue in any
invertebrate group, it is advisable to consider the other portion of
the cartilaginous skeleton of Ammoccetes, which consists of the tra-
becular, parachordals and auditory capsules — the basi-cranial skeleton
— and is composed of hard, not soft cartilage.

This basi-cranial skeleton represented in Fig. 53, B, is confined to
the region of the notochord, the cranial walls being composed entirely
of a white fibrous membrane. It is separated at first entirely from
the sub-chordal portion of the branchial basket-work, and is com-
posed of a foremost part, the trabecular (Tr.), and of a hindermost
part, the parachordals (Pr.ch.), which are characterized by the
attachment on each side of the large auditory capsule {Au.). In
Ammoccetes the trabecular bars are continuous with the parachordals,
the junction being marked by a small lateral projection on each side,
which at transformation is seen to play an important part in the
formation of the sub-ocular arch. The trabecular bar lies close
against the notochord on each side up to its termination ; it then
bends away from the middle line and curves round until it meets
its fellow on the opposite side, thus forming, as it were, the head of
a racquet of which the notochord forms the splice in the handle.
The strings of the racquet are represented by a thin membrane, in
the centre of which the position of the infundibulum {Inf.) of the

THE EVIDENCE OF THE SKELETON

13;

brain can be clearly seen. In an earlier stage of Ammoccetes the
two trabecular horns do not meet, but are separated by connective
tissue, which afterwards becomes cartilaginous.

As far, then, as the topography of this basi-cranial skeleton is
concerned, the striking points are — the shape of the trabecular
portion, diverging as it does around the infundibulum, and the pre-
sence on the parachordal portion of the two large auditory capsules.

These two points indicate, on the hypothesis that infundibulum
and oesophagus are convertible terms, that two supporting structures
of a cartilaginous nature must have existed in the ancestor of the
vertebrate, the first of which surrounded the oesophagus, and the
second was in connection with its auditory apparatus.

Structure of the Hard Cartilages.

The structure of this hard cartilage of the trabecular and auditory
capsules resembles that of the soft, in so far that it consists of large

Fig. 57. — A, Cartilage op Trabecule op Ammoccetes, stained with Hema-
toxylin and Picric Acid. B, Nests op Cartilage Cells in Entosternite
of Hypoctonus, stained with Hematoxylin and Picric Acid.

cells with a comparatively small amount of intercellular substance.
Schaffer, who has described it lately, considers that it is a nearer
approach to hyaline cartilage than the soft, but yet cannot be called
hyaline cartilage in the usual sense of the term. Its peculiarities
and its differences from the soft are especially well seen by its
staining reactions. I have myself been particularly struck with
the effect of picrocarmine or combined hseniatoxylin and picric acid

134 THE ORIGIN OF VERTEBRATES

staining (Fig. 57). In the case of the soft cartilage the capsular
substance stains respectively a brilliant red or blue, while that of
the hard cartilage is coloured a deep yellow, so that the junction
between the parachordals and the branchial cartilages is beautifully
marked out. Then, again, with thionin, which gives so marked a
reaction in the case of the soft cartilage, the hard cartilage of the
auditory capsule is not stained at all, and in the trabecule the deep
purple colour is confined to the mucoid cement-substance between
the capsules, just as Schaffer has stated. The same kinds of reactions
have been described by Schaffer: thus by double staining with
hrenialum-eosin the hard cartilage stains red, the soft blue ; and he
points out that even with over-staining by haemalum the auditory
capsule remains colourless, just as I have noticed with thionin. He
infers, precisely as I have done from the thionin reaction, that
chondro-mucoid, which is so marked a constituent of the soft cartilage
and of the muco-cartilage, is absent or present in but slight quantities
in the hard cartilage. Similarly, he points out that double staining
with tropceolin-methyl- violet stains the hard cartilage a bright orange
colour, and the soft cartilage a violet.

The evidence, then, shows clearly that a marked chemical differ-
ence exists between these two cartilages, which may be expressed by
saying that the one contains very largely a basophil substance,
which we may speak of as belonging to the class of chondro-mucoid
substances, while the other contains mainly an oxyphil substance,
probably a chondro-gelatine substance.

We may perhaps go further and attribute this difference of
composition to a difference of origin ; for whereas the soft cartilage
is invariably formed in a special tissue, the muco-cartilage, which
shows by its reaction how largely it is composed of a mucoid sub-
stance, the hard cartilage is certainly, in the case of the cartilage of
the cranium where its origin has been clearly made out, formed in
the membranous tissue of the cranium of Ammoccetes — i.e. in a
tissue which stains light blue with thionin, and contains a gelatinous
rather than a mucoid substratum.

The best opportunity of finding out the mode of origin of the
hard cartilage is afforded at the time of transformation, when so
much of this kind of cartilage is formed anew. Unfortunately, it
is very difficult to obtain the early transformation stages, conse-
cpuently we cannot be said to possess any really exhaustive and

THE EVIDENCE OF THE SKELETON 135

definite account of how the new cartilages are formed. Bujor,
Kaensche, and Schaffer all profess to give a more or less definite
account of their formation, and the one striking impression left on
the mind of the reader is how their descriptions vary. In one
point only are they agreed, and in that I also agree with them, viz.
the manner in which the new cranial walls are formed. Schaffer
describes the process as the invasion of chondroblasts into the
homogeneous fibrous tissue of the cranial walls. Such chondro-
blasts not only form the cartilaginous framework, but also assimilate
the fibrous tissue which they invade, so that finally all that remains
of the original fibrous matrix in which the cartilage was formed are
these lines of cement-substance between the groups of cartilage
cells, which, containing some basophil material, are marked out, as
already mentioned (Fig. 57).

We may therefore conclude, from the investigation of Ammoccetes,
that the front part of the basi-cranial skeleton arose as two trabecular
bars, to which muscles were attached, situated bilaterally with respect
to the central nervous system. These bars were composed of tendinous
material with a gelatinous rather than a mucoid substratum, in which
nests of cartilage- cells were formed, the cartilaginous material formed
by these cells being of the hard variety, not staining with thionin,
and staining yellow with picro-carmine, etc. By the increase of such
nests and the assimilation of the intermediate fibrous material, the
original fibro-cartilage was converted into the close-set semi-hyaline
cartilage of the trabecular and auditory capsules, in which the fibrous
material still marks out by its staining-reaction the limits of the
cell-clusters.

Such I gather to be Schaffer's conclusions, and they are certainly
borne out by my own and Miss Alcock's observations. As far as
we have had an opportunity of observing at present, the first process
at transformation appears to consist of the invasion of the fibrous
tissue of the cranial wall by groups of cells which form nests of cells
between the fibrous strands. These nests of cells form round them-
selves capsular material, and thus form cell-territories of cartilage,
which squeeze out and assimilate the surrounding fibrous tissue, until
at last all that remains of the original fibrous matrix is the lines of
cement-substance which mark out the limits of the various cell-groups.

At present I am inclined to think that both soft and hard cartilage
originate in a very similar manner, viz. by the formation of capsular

136 THE ORIGIN OF VERTEBRATES

material around the invading chondroblasts, and that the difference
in the resulting cartilage is mainly due to the difference in chemical
composition of the matrix of the connective tissue which is invaded.
Thus the difference may be formulated as follows : —

The hard cartilage is formed by the invasion of chondroblasts
into a fibrous tissue, which contains a gelatinous rather than a mucoid
substratum, in contradistinction to the soft cartilage which is formed,
probably also by the invasion of chondroblasts, in a tissue — the
muco-cartilage — which contains a specially mucoid substratum.

Such, then, is the very clearly defined starting-point of the ver-
tebrate skeleton — two distinct formations of different histological
and chemical structure,— the one forming a segmented branchial
skeleton, the other a non-segmented basi-cranial skeleton.

The Cartilaginous Skeleton of Limultjs.

Among the whole of the invertebrates at present living on the
earth, is there any sign of an internal cartilaginous skeleton that
will give a direct clue to the origin of the primitive vertebrate
skeleton ? The answer to this question is most significant : only
one animal among all those at present known possesses a cartilaginous
skeleton, which is directly comparable with that of Ammocoetes, and
here the comparison is very close — only one animal among the
thousands of living invertebrate forms, and that animal is the only
representative still surviving of the palseostracan group, which was
the dominant race when the vertebrate first made its appearance.
The Limulus, or king-crab, possesses a segmented branchial internal
cartilaginous skeleton (Fig. 53, A), made up of the same kind of cartilage
as the branchial skeleton of Ammocoetes, confined to the mesosomatic
or branchial region, just as in Ammocoetes, forming, as in Ammoccetes,
cartilaginous bars supporting the branchiae, and these bars are situated
externally to the branchiae, as in Ammocoetes. In addition this
animal possesses a basi-cranial internal semi-cartilaginous unseg-
mented plate known as the entosternite or plastron situated, with
respect to the oesophagus, similarly to the position of the trabecular
with respect to the infundibulum in Ammocoetes. Moreover, the
cartilaginous cells in this tissue differ from those in the branchial
region, in precisely the same manner as the hard cartilage differs from
the soft in Ammoccetes.

THE EVIDENCE OF THE SKELETON 1 37

This plastron, it is true, is found in other animals, all of which
are members of the scorpion tribe, except in one instance, and this,
strikingly enough, is the crustacean Apus — a strange primitive form,
which is acknowledged to be the nearest representative of the
Trilobita still living on the earth. None of these forms, however,
possess any sign of an internal cartilaginous branchial skeleton,
such as is possessed by Limulus. Scorpions, Apus, Limulus, are
all surviving types of the stage of organization which had been
reached in the animal world when the vertebrate first appeared.

The Mesosomatic oe Eespiratory Skeleton of Limulus, composed

of Soft Cartilage.

Searching through the literature of the histology of the cartila-
ginous tissues in invertebrate animals, to see whether any cartilage
had been described similar to that seen in the branchial cartilages of
Ammoccetes, and whether such cartilage, if found, arose in a fibrous
tissue resembling muco-cartilage, I was speedily rewarded by finding,
in Ray Lankester's article on the tropho- skeletal tissues of Limulus,
a picture of the cartilage of Limulus, which would have passed muster
for a drawing of the branchial cartilage of Ammoccetes. This clue
I followed out in the manner described in my former paper in the
Journal of Anatomy and Physiology, and mapped out the topography
of this remarkable tissue.

Limulus, like other water-dwelling arthropods, breathes by means
of gills attached to its appendages. These gill-bearing appendages
are confined to the mesosomatic region, as is seen in Fig. 59 ; and these
appendages are very different to the ordinary locomotor appendages,
which are confined to the prosomatic region. Each appendage, as is
seen in Fig. 58, consists mainly of a broad, basal part, which carries
the gill-book on its under surface ; the distal parts of the appendage
have dwindled to mere rudiments and still exist, not for locomotor
purposes, but because they carry on each segment organs of special
importance to the animal (see Chapter XL). As is seen in Fig. 58,
the basal parts of each pair of appendages form a broad, flattened
paddle, by means of which the animal is able to swim in a clumsy
fashion. Very striking and suggestive is the difference between
these gill-bearing mesosomatic appendages and the non-gill-bearing
locomotor appendages of the prosoina.

13^

THE ORIGIN OF VERTEBRATES

At the base of each of these appendages, where it is attached to
the body of the animal, the external chitinous surface is characterized

N. E/ULMS.

Fig. 58. — Transverse Section through the Mesosoma op Limulus, to show
the Anterior (A) and the Posterior (B) Surfaces of a Mesosomatic or
Branchial Appendage.

In each figure the branchial cartilaginous bar, Br.C, has been exposed by dissection
on one side. Ent., entapophysis ; Ent.l., entapophysial ligament cut across;
Br.C, branchial cartilaginous bar, which springs from the entapophysis ; H.,
heart; P., pericardium; Al., alimentary canal; N., nerve cord; L. V.S., longi-
tudinal venous sinus ; Dv., dorso-vencral somatic muscle; Vp., veno-pericardial
muscle.

by a peculiar stumpy, rod-like marking, and upon removing the
chitinous covering, this surface-appearance is seen to correspond to a
well-marked rod of cartilage (Br.C), which extends from the body

THE EVIDENCE OF THE SKELETON 1 39

of the animal well into each appendage. This bar of cartilage arises
on each side from the corresponding entapophysis (Ent.), which is
the name given to a chitinous spur which projects a short distance
(Fig. 58, B) into the animal from the dorsal side, for the purpose of
giving attachment to various segmental muscles. These entapophyses
are formed by an invagination of the chitinous surface on the dorsal
side and are confined to the mesosomatic region, so that the meso-
somatic carapace indicates, by the number of entapophyses, the
number of segments in that region, in contradistinction to the pro-
somatic carapace, which gives no indication on its surface of the
number of its components.

Each entapophysis is hollow and its walls are composed of chitin ;
but from the apex of each spur there stretches from spur to spur
a band of tissue, called by Lankester the entapophysial ligament
(Ent. I.) (Fig. 58), and in this tissue cartilage is formed. Isolated
cartilaginous cells, or rather groups of cells, are found here and there,
but a concentration of such groups always takes place at each enta-
pophysis, forming here a solid mass of cartilage, from which the
massive cartilaginous bar of each branchial appendage arises.

Further, not only is this cartilage exactly similar to parenchy-
matous cartilage, as it occurs in the branchial cartilages of Ammoccetes,
but also its matrix stains a brilliant purple with thionin in striking-
contrast to the exceedingly slight light-blue colour of the surrounding
perichondrium. In its chemical composition it shows, as might be
expected, that it is a cartilage containing a very large amount of
some mucin-body.

The Muco- cartilage of Limulus.

The resemblance between this structure and that of the branchial
bars of Ammoccetes does not end even here, for, as already mentioned,
the cartilage originates in a peculiar connective tissue band, the
entapophysial ligament, and this tissue bears the same relation in
its chemical reactions to the ordinary connective tissue of Limulus,
as muco-cartilage does to the white fibrous tissue of Ammocu'tes.
The white connective tissue of Limulus, as already stated, resembles
that of the vertebrate more than does the connective tissue of any
other invertebrate, and, similarly to that of Ammocn'tes, does not
stain, or gives only a light-blue tinge with thionin. The tissue of

140

THE ORIGIN OF VERTEBRATES

the entapophysial ligament, on the contrary, just like muco-cartilage,
takes on an intense purple colour when stained with thionin. It
possesses a mucoid substratum, just as does muco-cartilage, and in
both cases a perfectly similar soft cartilage is born from it.

One difference, however, exists between the branchial cartilages of
these two animals ; the innermost axial layer of the branchial bar of

Fig. 59. — Diagram of Limulus, to show the Nerves to the Appendages (1-13)

and the Branchial Cartilages.

The branchial cartilages and the entapophysial ligaments are coloured blue, the
branchise red. gl., generative and hepatic glands surrounding the central nervous
system and passing into the base of the flabellum (fl.).

Limulus is very apt to contain a specially hard substance, apparently
chalky in nature, so that it breaks up in sections, and gives the
appearance of a broken-down spongy mass ; if, however, the tissue is
first placed in a solution of hydrochloric acid, it then cuts easily, and
the whole tissue is seen to be of the same structure throughout, the
main difference being that the capsular spaces in the axial region
are much larger and much*more free from cell-protoplasm than are
those of the smaller younger cells near the periphery.

THE EVIDENCE OF THE SKELETON

I have attempted in Fig. 53 to represent this close resemblance
between the segmented branchial skeleton of Limulus and of Ammo-
cojtes, a resemblance so close as to reach even to minute details,
such as the thinning out of the cartilage in the subchordal bands and

Fig. 60. — Diagram of Ammoccetes cut open to show the Lateral System of
Cranial Nerves V., VII., IX., X., and the Branchial Cartilages.

The branchial cartilages and sub-chordal ligaments are coloured blue, the branchhe
red. (jl., glandular substance surrounding the central nervous system and pass-
ing into the auditory capsule with the auditory nerve (VIII).

entapophysial ligaments respectively between the places where the
branchial bars come off.

In Fig. 59 I have shown the prosoma and mesosoma of Limulus,
and indicated the nerves to the appendages together with the meso-
somatic cartilaginous skeleton.

In Fig. 60 I have drawn a corresponding picture of the prosomatic
and mesosomatic region of Aniuioccetes with the corresponding nerves

142 THE ORIGIN OF VERTEBRATES

and cartilages. In this figure the animal is supposed to be slit open
along the ventral mid-line and the central nervous system exposed.

The Prosomatic Skeleton of Limulus, composed of Hard

Cartilage.

The rest of the primitive vertebrate skeleton arose in the proso-
matic region, and formed a support for the base of the brain. This
skeleton was composed of hard cartilage, and arose in white fibrous
tissue containing gelatin rather than mucin.

Is there, then, any peculiar tissue of a cartilaginous nature in
Limulus and its allies, situated in the prosomatic region, which is
entirely separate from the branchial cartilaginous skeleton, which
acts as a supporting internal framework, and contains a gelatinous
rather than a mucoid substratum ?

It is a striking fact, common to the whole of the group of animals
to which our inquiries, deduced from the consideration of the structure
of Ammocoetes, have, in every case, led us in our search for the verte-
brate ancestor, that they do possess a remarkable internal semi-carti-
laginous skeleton in the prosomatic region, called the entosternite or
plastron, which gives support to a large number of the muscles of
that region ; which is entirely independent of the branchial skeleton,
and differs markedly in its chemical reactions from that cartilage, in
that it contains a gelatinous rather than a mucoid substratum.

In Limulus it is a large, tough, median plate, fibrous in character,
in which are situated rows and nests of cartilage-cells. The same
structure is seen in the plastron of Hypoctonus, of Thelyphonus,
and to a certainty in all the members of the scorpion group. Very
different is the behaviour of this tissue to staining from that of the
branchial region. No part of the plastron stains purple with
thionin ; it hardly stains at all, or gives only a very slight blue
colour. In its chemical composition there is a marked preponder-
ance of gelatin with only a slight amount of a mucin-body. In
some cases, as in Hypoctonus (Fig. 57, B) and Mygale, the capsules
of the cartilage-cells stain a deep yellow with ha^matoxylin and
picric acid, while the fibres between the cell-nests stain a blue-brown
colour, partly from the ha?matoxylin, partly from the picric acid.

All the evidence points to the plastron as resembling the basi-
cranial skeleton of Ammocoetes in its composition and in the origin

THE EVIDENCE OF THE SKELETON

14;

of its cells in a white fibrous tissue. What, then, is its topographical
position ? It is in all cases a median structure lying between the
cephalic stomach and the infra-< esophageal portion of the central
nervous system, and in all cases it possesses two anterior horns which
pass around the cesophagus and the nerve-masses which immediately
enclose the (esophagus (Fig. 61, A). These lateral horns, then,
which lie laterally and slightly ventral to the central nervous
system, and are called by Bay Lankester and Benham the sub-
neural portion of the entosternite, are very nearly in exactly the
position of the raccpuet- shaped head of the trabecuhe in Arnnioccetes.
It is easy to see that, with a more extensive growth of the nervous
material dorsally, such lateral
horns might be caused to take
up a still more ventral posi-
tion. Now, these two lateral
horns of the plastron of Li-
mulus are continued along
its whole length so as to form
two thickened lateral ridges,
which are conspicuous on the
flat surface of the rest of this
median plate. In other cases,
as in the Thelyphonida?, the
plastron consists mainly of
these two lateral ridges or
trabecuhe, as they might be
called, and Schimkewitsch,
who more than any one else has made a comparative study of the
entosternite, describes it as composed in these animals of two lateral
trabecular crossed by three transverse trabecule. I myself can con-
firm his description, and give in Fig. 61, B, the appearance of the
entosternite of Thelyphonus or of Hypoctonus. The supra-cesophageal
ganglia and part of the infra-cesophageal ganglia fill up the space Ph. ;
stretching over the rest of the infra-cesophageal mass is a transverse
trabecula, which is very thin ; then comes a space in which is seen
the rest of the infra-cesophageal mass, and then the posterior part of
the plastron, ventrally to which lies the commencement of the ventral
nerve-cord.

In these forms, in which the central nervous system is more

Fig. 61. — A, Entosternite of Limulus ;
B, Entosternite of Theta'phonus.

Ph., position of pharynx.

144 THE ORIGIN OF VERTEBRATES

concentrated towards the cephalic end than in Liniulus, the whole of
the concentrated brain-mass is separated from the gut only by this thin
transverse band of tissue. Judging, then, from the entosternite of
Thelyphonus, it is not difficult to suppose that a continuation of the
same growth of the brain-region of the central nervous system would
cause the entosternite to be separated into two lateral trabecular,
which would then take up the ventro-lateral position of the two
trabecular of Ammoccetes.

On the other hand, it might be that two lateral trabecular,
similar to those of Thelyphonus and situated on each side of the
central nervous system, were the original form from which, by the
addition of transverse fibres running between the gut and nervous
system, the entosternite of Thelyphonus and of the scorpions, etc.,
was formed. From an extensive consideration of the entosternite in
different animals, Schimkewitsch has come to the conclusion that this
latter explanation is the true one. He points out that the lateral
trabecules can be distinguished from the transverse by their structure,
being much more cellular and less fibrous, and the cell- cavities more
rounded, or, as I should express it, the two lateral trabecular are more
cartilaginous, while the transverse are more fibrous. Schimkewitsch,
from observations of structure and from embryological investi-
gations, comes to the conclusion that the entosternite was originally
composed of two parts —

1. A transverse muscle corresponding to the adductor muscle of
the shell of certain crustaceans, such as Nebalia.

2. A pair of longitudinal mesodermic tendons, which may have
been formed originally out of a number of segmen tally arranged
mesodermic tendons, and are crossed by the fibrils of the transverse
muscular bundles.

These paired tendons of the entosternite he considers to corre-
spond to the intermuscular tendons, situated lengthways, which are
found in the ventral longitudinal muscles of most arthropods.

It is clear from these observations of Schimkewitsch, that the
essential part of the entosternite consists of two lateral trabecular,
which were originally tendinous in nature and have become of the
nature of cartilaginous tissue by the increase of cellular elements in
the matrix of the tissue : these two trabecular function as supports
for the attachment of muscles, which are specially attached at
certain places. At these places transverse fibres belonging to some

THE EVIDENCE OF THE SKELETON 145

of the muscular attachments cross between the two longitudinal
trabecular, and so form the transverse trabecule.

I entirely agree with Schimkewitsch that the nests of cartilage-
cells are much more extensive in, and indeed nearly entirely
confined to, these two lateral trabecular in the entostemite of
Hypoctonus. Kay Lankester describes in the entostemite of Mygale
peculiar cell-nests strongly resembling those of Hypoctonus, and he
also states that they are confined to the lateral portions of the
entostemite.

From this evidence it is easy to see that that portion of the basi-
cranial skeleton known as the trabecular may have originated from
the formation of cartilage in the plastron or entostemite of a pake-
ostracan animal. Such an hypothesis immediately suggests valuable
clues as to the origin of the cranium and of the rest of the basi-
cranial skeleton — the parachordals and the auditory capsules. The
former would naturally be a dorsal extension of the more membranous
portion of the plastron, in which, equally naturally, cartilaginous tissue
would subsequently develop ; and the reason why it is impossible to
reduce the cranium into a series of segments would be self-evident,
for even though, as Schimkewitsch thinks, the plastron may have
been originally segmented, it has long lost all sign of segmentation.
The latter would be derived from a second entostemite of the same
nature as the plastron, but especially connected with the auditory
apparatus of the invertebrate ancestor. The following out of these
two clues will be the subject of a future chapter.

In our search, then, for a clue to the origin of the skeletal tissues
of the vertebrate we see again that we are led directly to the palaros-
tracan stock on the invertebrate side and to the Cyclostomata on that
of the vertebrate ; for in Limulus, the only living representative of
the Palaeostraca, and in Limulus alone, we find a skeleton marvel-
lously similar to the earliest vertebrate skeleton — that found in
Ammocoetes. Later on I shall give reasons for the belief that the
earliest fishes so far found, the Cephalaspidae, etc., were built up on
the same plan as Ammocoetes, so that, in my opinion, in Limulus
and in Ammocoetes we actually possess living examples allied to
the ancient fauna of the Silurian times.

146 THE ORIGIN OF VERTEBRATES

Summary.

The skeleton considered in this chapter is not the notochord, but that
composed of cartilage. The tracing- downwards of the vertebrate bony and
cartilaginous skeleton to its earliest beginnings leads straight to the skeleton of
the larval lamprey (Amnioccetes), in which vertebrae are not yet formed, but the
cranial and branchial skeleton is well marked.

The embryologies! and phylogenetic histories are in complete unison to show
that the cranial skeleton is older than the spinal, and this primitive branchial
skeleton is also in harmony with the laws of evolution, in that its structure, even
in the adult lamprey (Petromyzon). never gets beyond the stage characteristic
of embryonic cartilage in the higher vertebrates.

The simplest and most primitive skeleton is that found in Animoccetes and
consists of two parts : (1) a prosomatic, (2) a mesosomatic skeleton.

The prosomatic skeleton forms a non-segmented basi-cranial skeleton of the
simplest kind — the trabecular and the parachordals with their attached auditory
capsules, just as the embryology of the higher vertebrates teaches us must be
the case. There in the free-living, still-existent Ammoccetes we find the manifest
natural outcome of the embryological history in the shape of simple trabecular
and parachordals, from which the whole complicated basi-cranial skeleton of the
higher vertebrates arose.

The mesosomatic skeleton, which is formed before the prosoniatic, consisted,
in the first instance, of simple branchial bars segmentally arranged, which were
connected together by a longitudinal subchordal bar. situated laterally on each
side of the notochord. These simple branchial bars later on form the branchial
basket-work, which forms an open-work cage within which the branchiae are
situated.

The cartilages which compose these two skeletons respectively are markedly
different in chemical constitution, in that the first (hard cartilag'e) is mainly
composed of chondro-gelatin, the second (soft cartilage) of chondro-mucoid
material.

The same kind of difference is seen in the two kinds of connective tissue
which are the forerunners of these two kinds of cartilage. Thus, the cranial
walls in Ammoccetes are formed of white fibrous tissue, an essentially gelatin-
containing tissue ; at transformation these are invaded by chondro-blasts and
the cartilaginous cranium, formed of hard cartilage, results. On the other hand,
the forerunner of the branchial soft cartilage is a very striking and peculiar
kind of connective tissue loaded with mucoid material, to which the name
muco-cartilage has been given.

The enormous interest of this muco-cartilage consists in the fact that it
forms very well-defined plates of tissue, entirely confined to the head-region,
wliich are not found in any higher vertebrate, not even in the adult form
Petromyzon, for every scrap of the tissue as such disappears at transformation.

It is this evidence of primitive non-vertebrate tissues, which occur in the
larval but not in the adult form, which makes Ammoccetes so valuable for the
investigation of the origin of vertebrates.

The evidence, then, is extraordinarily clear as to the beginnings of the
vertebrate skeletal tissues.

THE EVIDENCE OF THE SKELETON 1 47

In the invertebrate kingdom true cartilage occurs but scantily. There is
a cartilaginous covering of the brain of cepkalopods. It is never found in crabs,
lobsters, bees, wasps, centipedes, butterflies, flies, or any of the great group of
Arthropoda, except, to a slight extent, in some members of the scorpion group,
aud more fully in one single animal, the King-crab or Limulus : a fact significant
of itself, but still more so when the nature of the cartilage and its position in
the animal is taken into consideration, for the identity both in structure and
position of this internal cartilaginous skeleton with that of Anmiocoetes is
extraordinarily g-reat.

Here, in Limulus. just as in Aminoccetes, an internal cartilaginous skeleton
is found, composed of two distinct parts : (1) prosomatic, (2) mesosomatic. As
in Ammocoetes, the latter consists of simple branchial bars, segmentally arranged,
which are connected together on each side by a longitudinal lig'ament contain-
ing cartilage — the entapophysial ligament. This cartilage is identical in
structure and in chemical composition with the soft cartilage of Ammocoetes,
and. as in the latter case, arises in a markedly mucoid connective tissue.
The former, as in Ammocoetes, consists of a non-segmental skeleton, the
plastron, composed of a white fibrous connective tissue matrix, an essentially
gelatin-containing tissue, in which are found nests of cartilage cells of the
hard cartilage variety.

This remarkable discovery of the branchial cartilaginous bars of Limulus,
together with that of the internal prosomatic plastron, causes the original diffi-
culty of deriving an animal such as the vertebrate from an animal resembling"
an arthropod to vanish into thin air, for it shows that in the past ages when the
vertebrates first appeared on the earth, the dominant arthropod race at that time,
the members of which resembled Limulus, had solved the question ; for, in addition
to their external chitinous covering, they had manufactured an internal cartila-
ginous skeleton. Not only so, but that skeleton had arrived, both in structure
and position, exactly at the stage at which the vertebrate skeleton starts.

What the precise steps are by which chitin-f ormation gives place to chondrin-
formation are not yet fully known, but Schmiedeberg has shown that a substance,
glycosamine, is derivable from both these skeletal tissues, and he concludes his
observations in the following words: ''Thus, by means of glycosamine, the
bridge is formed which connects together the chitin of the lower animals with
the cartilage of the more highly organized creations."

The evidence of the origin of the cartilaginous skeleton of the vertebrate
points directly to the origin of the vertebrate from the Palfeostraca, and is
of so' strong a character that, taken alone, it may almost be considered as proof
of such origin.

CHAPTEK IV

THE EVIDENCE OF THE RESPIRATORY APPARATUS

Branchiae considei*ed as internal branchial appendages. — Innervation of branchial
segments. — Cranial region older than spinal. — Three-root system of cranial
nerves, dorsal, lateral, ventral. — Explanation of van Wijhe's segments. —
Lateral mixed root is appendage-nerve of invertebrate. — The branchial
chamber of Ammocoetes. — The branchial unit, not a pouch but an
appendage. — The origin of the branchial musculature. — The branchial
circulation. — The branchial heart of the vertebrate. — Not homologous with
the systemic heart of the arthropod. — Its formation from two longitudinal
venous sinuses. — Summary.

The respiratory apparatus in all the terrestrial vertebrates is of the
same kind — one single pair of lungs. These lungs originate as a
diverticulum of the alimentary canal. On the other hand, the
aquatic vertebrates breathe by means of a series of branchiae, or gills,
which are arranged segmentally, being supported by the segmental
branchial cartilaginous bars, as already mentioned in the last chapter.
The transition from the gill-bearing to the lung-bearing vertebrates
is most interesting, for it has been proved that the lungs are formed
by the modification of the swim-bladder of fishes ; and in a group
of fishes, the Dipnoi, or lung-fishes, of which three representatives
still exist on the earth, the mode of transition from the -fish to the
amphibian is plainly visible, for they possess both lungs and
gills, and yet are not amphibians, but true fishes. But for the
fortunate existence of Ceratodus in Australia, Lepidosiren in South
America, and Protopterus in Africa, it would have been impossible
from the fossil remains to have asserted that any fish had ever
existed which possessed at the same moment of time the two kinds
of respiratory organs, although from our knowledge of the develop-
ment of the amphibian we might have felt sure that such a transitional
stage must have existed. Unfortunately, there is at present no
likelihood of any corresponding transitional stage being discovered

THE EVIDENCE OF THE RESPIRATORY APPARATUS 1 49

living on the earth in which both the dorsal arthropod alimentary
canal and the ventral vertebrate one should simultaneously exist in
a functional condition ; still it seems to me that even if Ceratodus,
Lepidosiren, and Protopterus had ceased to exist on the earth, yet
the facts of comparative anatomy, together with our conception of
evolution as portrayed in the theory of natural selection, would have
forced us to conclude rightly that the amphibian stage in the evolu-
tion of the vertebrate phylum was preceded by fishes which possessed
simultaneously lungs and gills.

In the preceding chapter the primitive cartilaginous vertebrate
skeleton, as found in Ammoccetes, was shown to correspond in a
marvellous manner to the cartilaginous skeleton of Limulus. In a
later chapter I will deal with the formation of the cranium from the
prosomatic skeleton ; in this chapter it is the mesosomatic skeleton
which is of interest, and the consideration of the necessary conse-
quences which logically follow upon the supposition that the branchial
cartilaginous bars of Limulus are homologous with the branchial
basket-work of Ammoccetes.

Internal Branchial Appendages.

Seeing that in both cases the cartilaginous bars of Limulus and
Ammoccetes are confined to the branchial region, their homology of
necessity implies an homology of the two branchial regions, and leads
directly to the conclusion that the branchiae of the vertebrate were
derived from the branchiae of the arthropod, a conclusion which,
according to the generally accepted view of the origin of the respira-
tory region in the vertebrate, is extremely difficult to accept ; for the
branchial of Limulus and of the Arthropoda in general are part of
the mesosomatic appendages, while the branchiae of vertebrates are
derived from the anterior part of the alimentary canal. This con-
clusion, therefore, implies that the vertebrate lias utilized in the
formation of the anterior portion of its new alimentary canal the
branchial appendages of the palasostracan ancestor.

Let us consider dispassionately whether such a suggestion is a priori
so impossible as it at first appears. One of the principles of evolution
is that any change which is supposed to have taken place in the
process of formation of one animal or group of animals from a lower
group must be in harmony with changes which are known to have

i5o

THE ORIGIN OF VERTEBRATES

occurred in that lower group. On the assumption, therefore, that
the vertebrate branchiae represent the branchial portion of the
arthropod mesosomatic appendages which have sunk in and so
become internal, we ought to find that in members of this very
group such inclusion of branchial appendages has taken place. This,
indeed, is exactly what we do find, for in all the scorpion tribe, which
is acknowledged to be closely related to Limulus, there are no

external mesosomatic appendages,
but in all cases these appendages
have sunk into the body, have
disappeared as such, and retained
only the vital part of them — the
branchiae. In this way the so-called
lung - books of the scorpion are
formed, which are in all respects
homologous with the branchiae or
gill-books of Limulus. Now, as
already mentioned, the lords of
creation in the palseostracan times
were the sea-scorpions, which, as is
seen in Fig. 62, resembled the land-
scorpions of the present day in the
entire absence of any external ap-
pendages on the segments of the
mesosomatic region. As they lived
in the sea, they must have breathed
with gills, and those branchial ap-
pendages must have been internal,
just as in the land-scorpions of the
present time. Indeed, markings
have been found on the internal
side of the segments 1-5, Fig. 62, which are supposed to indicate
branchiffi, and these segments are therefore supposed to have borne
the branchire. Up to the present time no indication of gill-slits
has been found, and we cannot say with certainty how these
animals breathed. Further, in the Upper Silurian of Lesmahago,
Lanarkshire, a scorpion (Palccoijhonus Hunteri), closely resembling the
modern scorpion, has been found, which, as Lankester states, was in
all probability aquatic, and not terrestrial in its habits. How it

Fig. 62. — Eurypterus.

The segments and appendages on the
right are numbered in correspon-
dence with the cranial system of
lateral nerve-roots as found in verte-
brates. lf.,metastoma. The surface
ornamentation is represented on
the first segment posterior to the
branchial segments. The opercular
appendage is marked out by dots.

THE EVIDENCE OF THE RESPIRATORY APPARATUS 15 I

breathed is unknown ; it shows no signs of stigmata, such as exist in
the scorpion of to-day.

Although we possess as yet no certain knowledge of the position
of the gill-openings in these ancient scorpion-like forms, what we
can say with certainty — and that is the important fact — is, that at
the time when the vertebrates appeared, a very large number of the
dominant arthropod race possessed internally-situated branchife, which
had been directly derived from the branchiae-bearing appendages of
their Limulus-like kinsfolk.

This abolition of the branchiie-bearing appendages as external
organs of locomotion, with the retention of the important branchial
portion of the appendage as internal branchiae, is a very important
suggestion in any discussion of the way vertebrates have arisen from
arthropods; for, if the same principle is of universal application, it
leads directly to the conclusion that whenever an appendage possesses
an organ of vital importance to the animal, that organ will remain,
even though the appendage as such completely vanishes. Thus, as
will be shown later, special sense-organs such as the olfactory remain,
though the animal no longer possesses antennae ; the important ex-
cretory organs, the coxal glands, and important respiratory organs,
the branchiae, are still present in the vertebrate, although the appen-
dages to which they originally belonged have dwindled away, or, at
all events, are no longer recognizable as arthropod appendages.

Innervation of Beanchial Segments.

Passing from a priori considerations to actual facts, it is advisable
to commence with the innervation of the branchial segments ; for,
seeing that the foundation of the whole of this comparative study
of the vertebrate and the arthropod is based upon the similarity of
the two central nervous systems, it follows that we must look in
the first instance to the innervation of any organ or group of organs
in order to find out their relationship in the two groups of animals.

The great characteristic of the vertebrate branchial organs is their
segmental arrangement and their innervation by the vagus group of
nerves, i.e. by the hindermost group of the cranial segmental nerves.
These cranial nerves are divided by Gegenbaur into two great groups
— an anterior group, the trigeminal, which supplies the muscles of
mastication, and a posterior group, the vagus, which is essentially

152 THE ORIGIN OF VERTEBRATES

respiratory in function. Of these two groups, I will consider the
latter group first.

In Limulus the great characteristic of the branchial region is its
oronounced segmental arrangement, each pair of branchial appendages
belonging to a separate segment. This group of segments forms the
mesosoma, and these branchial appendages are the mesosomatic
appendages. Anterior to them are the segments of the prosoma,
which bear the prosomatic or locomotor appendages. The latter are
provided at their base with gnathites or masticating apparatus, so
that the prosomatic group of nerves, like the trigeminal group in the
vertebrate, comprises essentially the nerves subserving the important
function of mastication. As already pointed out, the brain-region
of the vertebrate is comparable to the supra-cesophageal and infra-
cesophageal ganglia of the invertebrate, and it has been shown (p. 54)
how. by a process of concentration and cephalization, the foremost
region of the infra-cesophageal ganglia becomes the prosomatic region,
and is directly comparable to the trigeminal region in the vertebrate ;
while the hindermost region is formed from the concentration of
the mesosomatic ganglia, and is directly comparable to the medulla
oblongata, i.e. to the vagus region of the vertebrate brain.

As far, then, as concerns the centres of origin of these two groups
of nerves and their exits from the central nervous system, they are
markedly homologous in the two groups of animals.

Comparison of the Cranial and Spinal Segmental Nerves.

It has often been held that the arrangements of the vertebrate
nervous system differ from those of other segmented animals in one
important particular. The characteristic of the vertebrate is the
origin of every segmental nerve from two roots, of which one con-
tains the efferent fibres, while the other possesses a sensory ganglion,
and contains only afferent fibres. This arrangement, which is found
along the whole spinal cord of all vertebrates, is not found in the
segmental nerves of the invertebrates ; and as it is supposed that the
simpler arrangement of the spinal cord was the primitive arrange-
ment from which the vertebrate central nervous system was built up,
it is often concluded that the animal from which the vertebrate arose
must have possessed a series of nerve-segments, from each of which
there arose bilaterally ventral (efferent) and dorsal (afferent) roots.

THE EVIDENCE OE THE RESPIRATORY APPARATUS 1 53

Now, the striking fact of the vertebrate segmental nerves consists
in this, that, as far as their structure and the tissues which they
innervate are concerned, the cranial segmental nerves are built up on
the same plan as the spinal ; but as far as concerns their exit from
the central nervous system they are markedly different. A large
amount of ingenuity, it is true, has been spent in the endeavour to
force the cranial nerves into a series of segmental nerves, which
arise in the same way as the spinal by two roots, of which the ven-
tral series ought to be efferent and the dorsal series afferent, but
without success. We must, therefore, consider the arrangement of
the cranial segmental nerves by itself, separately from that of the
spinal nerves, and the problem of the origin of the vertebrate seg-
mental nerves admits of two solutions — either the cranial arrange-
ment has arisen from a modification of the spinal, or the spinal from
a simplification of the cranial. The first solution implies that the
spinal cord arrangement is older than the cranial, the second that
the cranial is the oldest.

In my opinion, the evidence of the greater antiquity of the cranial
region is overwhelming.

The evidence of embryology points directly to the greater phylo-
genetic antiquity of the cranial region, for we see how, quite early in
the development, the head is folded off, and the organs in that
region thereby completed at a time when the spinal region is only at
an early stage of development. We see how the first of the trunk
somites is formed just posteriorly to the head region, and then more
and more somites are formed by the addition of fresh segments poste-
riorly to the one first formed. We see how, in Ammoccetes, the first
formed parts of the skeleton are the branchial bars and the basi-
cranial system, while the rudiments of the vertebra? do not appear
until the Petromyzon stage. We see how, with the elongation of the
animal by the later addition of more and more spinal segments,
organs, such as the heart, which were originally in the head, travel
down, and the vagus and lateral-line nerves reach their ultimate
destination. Again, we see that, whereas the cranial nerves, viz. the
ocular motor, the trigeminal, facial, auditory, glossopharyngeal, and
vagus nerves, are wonderfully fixed and constant in all vertebrates,
the only shifting being in the spino-occipital region, in fact, at the
junction of the cranial and spinal region, the spinal nerves, on the
other hand, are not only remarkably variable in number in different

154 THE ORIGIN OF VERTEBRATES

groups of animals, but that even in the same animal great variations
are found, especially in the manner of formation of the limb-plexuses.
Such marked meristic variation in the spinal nerves, in contrast to
the fixed character of the cranial nerves, certainly points to a more
recent formation of the former nerves.

Also the observations of Assheton on the primitive streak of the
rabbit, and on the growth in length of the frog embryo, have led
him to the conclusion that, as in the rabbit so in the frog, there
is evidence to show that the embryo is derived from two definite
centres of growth : the first, phylogenetically the oldest, being a
protoplasmic activity, which gives rise to the anterior end of the
embryo ; the second, one which gives rise to the growth in length of
the embryo. This secondary area of proliferation coincides with the
area of the primitive streak, and he has shown, in a subsequent
paper, by means of the insertion of sable hairs into the unincubated
blastoderm of the chick, that a hair inserted into the centre of the
blastoderm appears at the anterior end of the primitive streak, and
subsequently is found at the level of the most anterior pair of somites.

He then goes on to say —

"From these specimens it seems clear that all those parts in
front of the first pair of mesoblastic somites — that is to say, the
heart, the brain and medulla oblongata, the olfactory, optic, auditory
organs and foregut — are developed from that portion of the un-
incubated blastoderm which lies anterior to the centre of the blasto-
derm, and that all the rest of the embryo is formed by the activity
of the primitive streak area."

In other words, the secondary area of growth, i.e. the primitive
streak area, includes the whole of the spinal cord region, while the
older primary centre of growth is coincident with the cranial region.

In searching, then, for the origin of the segmental nerves, we
must consider the type on which the cranial nerves are arranged
rather than that of the spinal nerves.

The first striking fact occurs at the spino-occipital region, where
the spinal cord merges into the medulla oblongata, for here in the
cervical region we find each spinal segment gives origin to three dis-
tinct roots, not two — a dorsal root, a ventral root, and a lateral root.
This third root gives origin to the spinal accessory nerve, and in the
region of the medulla oblongata these lateral roots merge directly
into the roots of the vagus nerve; more anteriorly the same system

THE EVIDENCE OF THE RESPIRATORY APPARATUS 1 55

continues as the roots of the glossopharyngeal nerve, as the roots of
the facial nerve, and as a portion, especially the motor portion, of
the trigeminal nerve. Now, all these nerves belong to a well-defined
system of nerves, as Charles Bell1 pointed out in 1830, a system of
nerves concerned with respiration and allied mechanisms, such as
laughing, sneezing, mastication, deglutition, etc., nerves innervating a
set of muscles of very different kind from the ordinary body-muscles
concerned with locomotion and equilibration. Also the centres from
which these motor nerves arise are well defined, and form cell-masses
in the central nervous system, quite separate from those which give
origin to somatic muscles.

This original idea of Charles Bell, after having been ignored for so
long a time, is now seen to be a very right one, and it is an extra-
ordinary thing that his enunciation of the dual nature of the spinal
roots, which was, to his mind, of subordinate importance, should so
entirely have overshadowed his suggestion, that in addition to the
dorsal and ventral roots, a lateral system of nerves existed, which
were not exclusively sensory or exclusively motor, but formed a
separate system of respiratory nerves.

Further, anatomists divide the striated muscles of the body into
two great natural groups, characterized by a difference of origin and
largely by a difference of appearance. The one set is concerned
with the movements of internal organs, and is called visceral, the
other is derived from the longitudinal sheet of musculature which
forms the myotomes of the fish, and has been called parietal or
somatic. The motor nerves of these two sets of muscles correspond
with the lateral or respiratory and ventral roots respectively.

Finally, it has been shown that the segments of which a verte-
brate is composed are recognizable in the embryo by the segmented
manner in which the musculature is laid down, and van Wijhe has
shown that in the cranial region two sets of muscles are laid down
segmentally, thus forming a dorsal and ventral series of commencing
muscular segments. Of these the anterior segments of the dorsal
series give origin to the striated muscles of the eye which are inner-
vated by the Illrd (oculomotor), IVth (trochlearis), and Vlth (ab-
ducens) nerves, while the posterior segments give origin to the

1 N.B. — In addition to the nerves mentioned, C. Bell included, in his respiratory
system of nerves, the fourth nerve or trochlearis, the phrenic and the external
respiratory of Bell.

i56

THE ORIGIN OF VERTEBRATES

muscles from the cranium to the shoulder-girdle, innervated by the
Xllth (hypoglossal) nerve. The ventral series of segments give
origin to the musculature supplied by the trigeminal, facial, glosso-
pharyngeal, and vagus nerves.

Also, the afferent or sensory nerves of the skin over the whole of
this head-region are supplied by the trigeminal nerve, while the
afferent nerves to the visceral surfaces are supplied by the vagus,
glossopharyngeal and facial nerves.

In van "Wijhe's original paper he arranged the segments belonging
to the cranial nerves in the following table : —

Segment?.

Ventral nerve-roots and muscles
derived from myotomes.

Visceral clefts.

Dorsal nerve-roots and muscles.

M. rectus supe-
rior, m. rectus
internus, m.
rectus inferior,
m. obliquus in-

V. N.op-
thalrnicus
profundus

ferior

IV.

M. obliquus

Masticating

superior

1st Mandibular

muscles.

VI.

M. rectus ex-

VTL,

i Facial muscles
(VIII. is dorsal

—

ternum

^{I$£

VII.,

1 branch of VII.)

—

3rd 1st Branchial

IX.

XII.

xn.

j Muscles from j
cranium to I

4th 2nd

5th 3rd „

6th 4th

X.,
X...

x.:

Branchial and
visceral muscles

XII.

1 shoulder-girdle |

7th 5th

x.t

As is seen in the table, van Wijhe attempts to arrange the cranial
secrmental nerves into dorsal and ventral roots, in accordance with
the arrangement in the spinal region. In order to do this he calls
the Vth. Vllth, IXth, and Xth nerves dorsal roots, although they
are not purely sensory nerves, but contain motor fibres as well.

It is not accidental that he should have picked out for his dorsal
roots the very nerves which form Charles Bell's lateral series of
roots, inasmuch as this system of lateral roots, apart from dorsal and
ventral roots, really is, as Charles Bell thought, an important separate
system, dependent upon a separate segmentation in the embryo of
the musculature supplied by these roots. This segmentation may
receive the name of visceral or splanchnic in contradistinction to
somatic, since all the muscles without exception belong to the visceral
group of striated muscles.

THE EVIDEXCE OF THE RESPIRATORY APPARATUS 1 57

These observations of van Wijhe lead directly to the following
conclusion. In the cranial region there is evidence of a double set
of segments, which mav be called somatic and splanchnic. The
somatic segments, consisting of the outer skin and the body muscu-
lature, are doubly innervated as are those of the spinal cord by a
series of ventral motor roots, the oculomotor or lllrd nerve, the
trochlear or IVth nerve, the abducens or YIth nerve, and the hypo-
glossal or Xllth nerve, and by a series of dorsal sensory roots, the
sensory part of the trigeminal or Yth nerve. But the splanchnic
segments are innervated by single roots, the vagus or Xth nerve,
glossopharyngeal or IXth nerve, facial or Vllth nerve, and trigeminal
or Vth nerve, which are mixed, containing both sensory and motor
fibres, thus differing markedly from the arrangement of the spinal
nerves.

From this sketch it follows that the arrangement seen in the
spinal cord, would result from the cranial arrangement if this third
system of lateral roots were left out. Further, since the cranial
system is the oldest, we must search in the invertebrate ancestor for
a tripartite rather than a dual system of nerve-roots for each segment ;
a system composed of a dorsal root supplying only the sensory nerves
of the skin-surfaces, a lateral mixed root supplying the system con-
nected with respiration with both sensory and motor fibres, and a
ventral root supplying the motor nerves to the body-musculature.

COMPARISON OF THE APPENDAGE NERVES OF LlMULUS AND BrANCHI-

pus to the Lateral Eoot System of the Vertebrate.

If the argument used so far is correct, and this tripartite system
of nerve-roots, as seen in the cranial nerves of the vertebrate, really
represents the original scheme of innervation in the paheostracan
ancestor, then it follows that each segment of Limulus ought to be
supplied by three nerves— (1), a sensory nerve supplying its own
portion of the skin-surface of the prosomatic and mesosumatic
carapaces; (2), a lateral mixed nerve supplying exclusively the
appendage of the segment, for the appendages carry the respiratory
organs ; and (3), a motor nerve supplying the body -muscles of the
segment.

It is a striking fact that Milne-Edwards describes the nerve-roots
in exactly this manner. The great characteristic v£ the nerve-roots

158 THE ORIGIN OF VERTEBRATES

in Limulus as in other arthropods is the largo appendage-nerve,
which is always a mixed nerve; in addition, there is a system of
sensory nerves to the prosomatic and mesosomatic carapaces, called
by him the epimeral nerves, which are purely sensory, and a third
set of roots which are motor to the body-inuseles, and possibly also
sensory to the ventral surface between the appendages.

Moreover, just as in the vertebrate central nervous system the
centres of origin of the motor nerves of the branchial segmentation
are distinct from those of the somatic segmentation, so we find, from
the researches of Hardy, that a similar well-marked separation exists
between the centres of origin of the motor nerves of the appendages
and those of the somatic muscles in the central nervous system of
Branchipus and Astacus.

In the first place, he points out that the nervous system of
Branchipus is of a very primitive arthropod type ; that it is, in fact,
as good an example of an ancient type as we are likely to find in the
present day ; a matter of some importance in connection with my
argument, since the arthropod ancestor of the vertebrate, such as I
am deducing from the study of Ammoccetes, must undoubtedly have
been of an ancient type, more nearly connected with the strange
forms of the trilobite era than with the crabs and spiders of the
present day.

His conclusions with respect to Branchipus may be tabulated as
follows : —

1. Each ganglion of the ventral chain is formed mainly for the
innervation of the appendages.

2. Each ganglion is divided into an anterior and posterior division,
which are connected respectively with the motor and sensory nerves
of the appendages.

3. The motor nerves of the appendages arise as well-defined axis-
cylinder processes of nerve-cells, which are arranged in well-defined
groups in the anterior division of the ganglion.

4. A separate innervation exists for the muscles and sensory
surfaces of the trunk. The trunk-muscles consist of long bundles,
from which slips pass off to the skin in each segment; they are thus
imperfectly segmented. In accordance with this, a diffuse system
of nerve-fibres passes to them from certain cells on the dorsal surface
of each lateral half of the ganglion. These cell-groups are therefore
very distinct from those which give origin to the motor appendage-

THE EVIDENCE OF THE RESPIRATORY APPARATUS 1 59

nerves, and, moreover, are not confined to the ganglion, but extend
for some distance into the interganglionic region of the nerve-cords
which connect together the ganglia of the ventral chain.

Hardy's observations, therefore, combined with those of Milne-
Edwards, lead to the conclusion that in such a primitive arthropod
type as my theory postulates, each segment was supplied with
separate sensory and motor somatic nerves, and with a pair of nerves
of mixed function, devoted entirely to the innervation of the pair of
appendages ; that also, in the central nervous system, the motor
nerve-centres were arranged in accordance with a double set of seg-
mented muscles in two separate groups of nerve-cells. These nerve-
cells in the one case were aggregated into well-defined groups, which
formed the centres for the motor nerves of the markedly segmented
muscles of the appendages, and in the other case formed a system of
more diffused cells, less markedly aggregated into distinct groups,
which formed the centres for the imperfectly segmented somatic
muscles.

Such an arrangement suggests that in the ancient arthropod type
a double segmentation existed, viz. a segmentation of the body, and
a segmentation due to the appendages. Undoubtedly, the segments
originally corresponded absolutely as in Branchipus, and every
appendage was attached to a well-defined separate body-segment.
In, however, such an ancient type as Limulus, though the segmen-
tation may be spoken of as twofold, yet the number of segments
in the prosoinatic and mesosomatic regions are much more clearly
marked out by the appendages than by the divisions of the soma ;
for, in the prosoinatic region such a fusion of somatic segments
to form the tergal prosoinatic carapace has taken place that the
segments of which it is composed are visible only in the young con-
dition, while in the mesosomatic region the separate somatic segments,
though fused to form the mesosomatic carapace, are still indicated
by the entapophysial indentations.

Clearly, then, if the mesosomatic branchial appendages of forms
related to Limulus were reduced to the branchial portion of the
appendage, and that branchial portion became internal, just as is
known to be the case in the scorpion group, we should obtain an
animal in which the mesosomatic region would be characterized by
a segmentation predominantly branchial, which might be termed, as
in vertebrates, the oranchiomcric segmentation, but yet would show

160 THE ORIGIN OF VERTEBRATES

indications of a corresponding somatic or mesomeric segmentation.
The nerve supply to these segments would consist of —

1. The epimeral purely sensory nerves to the somatic surface,
equivalent in the vertebrate to the ascending root of the trigeminal.

2. The mixed nerves to the internal branchial segments, equivalent
in the vertebrate to the vagus, glossopharyngeal, and facial.

3. The motor nerves to the somatic muscles, equivalent in the
vertebrate to the original nerve- supply to the somatic muscles
belonging to these segments, i.e. to the muscles derived from van
Wijhe's 4th, 5th, and 6th somites.

Further, the centres of origin of these appendage-nerves would
form centres in the central nervous system separate from the centres
of the motor nerves to the somatic muscles, just as the centres of
origin of the motor parts of the facial, vagus, and glossopharyngeal
nerves form groups of cells quite distinct from the centres for the
hypoglossal, abducens, trochlear, and oculomotor nerves.

In fact, if the vertebrate branchial nerves are looked upon as the
descendants of nerves which originally supplied branchial appendages,
then every question connected with the branchial segmentation, with
the origin and distribution of these nerves, receives a simple and
adequate solution — a solution in exact agreement with the conclusion
that the vertebrate arose from a pakeostracan ancestor.

It would, therefore, be natural to expect that the earliest fishes
breathed by means of branchial appendages situated internally, and
that the evidence for such appendages would be much stronger in
them than in more recent fishes.

Although we know nothing of the nature of the respiratory appa-
ratus in the extinct fishes of Silurian times, we have still living, in
the shape of Ammoccetes, a possible representative of such types.
If, then, we find, as is the case, that the respiratory apparatus of
Ammocoetes differs markedly from that of the rest of the fishes, and,
indeed, from that of the adult form or Petromyzon, and that that
very difference consists in a greater resemblance to internal branchial
appendages in the case of Ammoccetes, then we may feel that the
proof of the origin of the branchial apparatus of the vertebrate from
the internal branchial appendages of the invertebrate has gained
enormously.

THE EVIDENCE OF THE RESPIRATORY APPARATUS l6l

The Eespiratory Chamber of Ammoccetes.

In order to make clear the nature of the branchial segments in
Ammoco?tes, I have divided the head-part of the animal by means of
a longitudinal horizontal section into halves — ventral and dorsal —
as shown in Figs. 63 and 64. These figures are each a combination
of a section and a solid drawing. The animal was slit open by a
longitudinal section in the neighbourhood of the gill-slits, and each
half was slightly flattened out, so as to expose the ventral and dorsal
internal surfaces respectively. The structures in the cut surface were
drawn from one of a series of horizontal longitudinal sections taken
through the head of the animal. These figures show that the head-region
of Ammoccetes consists of two chambers, the contents of which are
different. In front, an oral or stomodseal chamber, which contains the
velum and tentacles, is enclosed by the upper and lower lips, and was
originally separated by a septum from the larger respiratory chamber,
which contains the separate pairs of branchiae. A glance at the two
drawings shows clearly that Eathke's original description of this
chamber is the natural one, for he at that time, looking upon Ammo-
ccetes branchialis as a separate species, described the branchial chamber
as containing a series of paired gills, with the gill-openings between
consecutive gills. His branchial unit or gill, therefore, was repre-
sented by each of the so-called diaphragms, which, as seen in Figs. 63,
64, are all exactly alike, except the first and the last. Any one of
these is represented in section in Fig. 65, and represents a branchial
unit in Eathke's view and in mine. Clearly, it may be described as a
branchial appendage which projects into an open pharyngeal chamber,
so that the series of such appendages divides the chamber into a
series of compartments, each of which communicates with the exterior
by means of a gill-slit, and with each other by means of the open
space between opposing appendages.

Each of these appendages possesses its own cartilaginous bar
(Br. cart.), as explained in Chapter III. ; each possesses its own bran-
chial or visceral muscles (coloured blue in Figs. 63 and 64), separated
absolutely from the longitudinal somatic muscles (coloured dark
red in Figs. 63 and 64) by a space (*S^>.) containing blood and
peculiar fat-cells, etc, Each possesses its own afferent branchial
blood-vessel from the ventral aorta, and its own efferent vessel to
the dorsal aorta (Fig. 65, a. br. and v. br.). Each possesses its own

Respiratory Append aqes
$ Nerve Supply

Huoiti

Fig. 63. — Ventral half
of Head-region of Am-
moccetes.

-Pigment

Somatic muscles coloured
red. Branchial and visce-
ral muscles coloured blue.
Tubular constrictor mus-
cles distinguished from
striated constrictor mus-
cles by simple hatching.
Tent., tentacles ; Tent. m.c.,
muco-cartilage of tenta-
cles; Vel. m.c, muco-car-
tilage of the velum ; Hy.
m.c. muco-cartilage of the
hyoid segment; Ps. br.,
pseudo-branchial groove ;
Br. cart., branchial carti-
lages ; Sp., space between
somatic and splanchnic
muscles ; Tit. op., orifice of
thyroid ; //., heart.

Tr.

<Ser.

Fig. 64. — Dorsal
half of head-

REGIOV OF Am-
MOCOiTES.

Inf.

Tr., trabecule;
Pit., pituitary

space ; //;/"., in-
f u n d i b u 1 u m ;
Ser., median ser-
rated flange of
velar folds.

164

THE ORIGIN OF VERTEBRATES

segmental nerve, which supplies its own- branchial muscles and no
others with motor fibres, and sends sensory fibres to the general surface
of each appendage, as also to the special sense-organs in the shape
of the epithelial pits (S., Fig. 65) arranged along the free edges of

m.add

v br.cart

m,:

m.v

Fig. 65. — Section through Branchial Ap-
pendage of Ammoccetes.

br. cart., branchial cartilage; v. br., branchial
vein; a. br., branchial artery; b.s., blood-
spaces ; p., pigment ; 8., sense-organ; c, cili-
ated band; E., I., external and internal
borders ; m. add., adductor muscle ; m.c.s.,
striated constrictor muscle; m.c.t., tubular
constrictor muscle ; m. and m.v., muscles
of valve.

br.cart.

Fig. 66. — Section through Bran-
chial Appendage of Limulus.

br. cart., branchial cartilage ;
v.br., branchial vein ; b.s., blood-
spaces formed by branchial artery ;
P., pigment ; nti, posterior enta-
pophysio-branchial muscle ; m„,
anterior entapophysio-branchial
muscle ; w3, external branchial
muscle.

the diaphragms ; each of these nerves possesses its own ganglion —
the epibranchial ganglion.

The work of Miss Alcock has shown that the segmental branchial
nerve supplies solely and absolutely such an appendage or branchial

THE EVIDENCE OF THE RESPIRATORY APPARATUS 1 65

segment, and does not supply any portion of the neighbouring branchial
segments. The nerve-supply in Ammoccetes gives no countenance to
the view that the original unit was a branchial pouch, the two sides
of which each nerve supplied, but is strong evidence that the original
unit was a branchial appendage, which was supplied by a single
nerve with both motor and sensory fibres.

Any observer having before him only this picture of the respiratory
chamber of Ammoccetes, upon which to base his view of a vertebrate
respiratory chamber, would naturally look upon the branchial unit of
a vertebrate as a gilled appendage projecting into the open cavity of
the anterior part of the alimentary canal or pharynx. This is not,
however, the usual conception. The branchial unit is ordinarily
described as a gill-pouch, which possesses two openings or slits, an
internal one into the lumen of the alimentary canal, and an external
one into the surrounding medium. This view is based upon embryo-
logical evidence of the following character : —

The alimentary canal of all vertebrates forms a tube stretching
the whole length of the animal ; the anterior part of this tube
becomes pouched on each side at regular intervals, and the walls of
each pouch becoming folded form the respiratory surfaces or gills.
The openings of these separate pouches into the central lumen of the
gut form the internal gill-pouch openings ; the other extremity of
the pouch approaches the external surface of the animal, and finally
breaks through to form a series of external gill-pouch openings.

From the mesoblastic tissue, between each gill-pouch, there is
formed a supporting cartilaginous bar, to which are attached a system
of branchial muscles, with their nerves and blood-vessels. These
cartilaginous bars, in all fishes above the Cyclostomata, form a
supporting framework for the internal gill-slit, so that the gills
are situated externally to them ; the more primitive arrangement is,
as already mentioned, a system of cartilaginous bars, extra-branchial
in position, so that the gills are situated internally to them.

From this description of the mode of formation of the respiratory
apparatus in water-breathing vertebrates the conception has arisen
of the gill-pouch as the branchial unit, a conception which is
absolutely removed from all idea of a branchial unit such as is
found in an arthropod, viz. an appendage.

This conception of spaces as units pervades the whole of embryo-
logy, and is the outcome of the gastrula theory— a theory which

1 66

THE ORIGIN OF VERTEBRATES

teaches that all animals above the Protozoa are derived from a form
which by invagination of its external surface formed an internal
cavity or primitive gut. From pouches of this gut other cavities
were said to be formed, called coelomic cavities, and thus arose the
group of cceloinatous animals. To speak of the developmental history
of animals in terms of spaces ; to speak of the atrophy of a cavity
as though such a thing were possible, is, to my mind, the wrong
way of looking at the facts of anatomy. It resembles the description
of a net as a number of holes tied together with string, which is not
usually considered the best method of description.

There are two ways in which a series of pouches can be formed
from a simple tube without folding, either by a thinning at regular
intervals of the original tissue surrounding the tube, or by the
ingrowth into the tube of the surrounding tissue at regular intervals,
thus —

Ep -
Mes-

ODOOOQQQQQDCDC03CG00aQO0Q3

\ it; t 1 1 rr-

OQSDODDaDCDDDaDQOSaOSaDDODBa

'SEGMENT' ;

araQoaQQaaQDaQQbgoQoaaoDOQOSDO

Ep-

Mes-

CDCKBCCCDDDBODCOBGDBDOaDDD.

DDanuuoauauaaDnanaaD

QGCOEBDaCDa00aBQDDBO0DQ3BQQag

Fig. 67.

1 z

-Diagrams to show the two methods of Pouch-formation.

A, by the thinning of the mesoblast at intervals. B, by the ingrowth of rnesoblast at
intervals. Ep., epiblast ; Mes., mesoblast ; Hy., hypoblast.

In the first case (A) the formation of a pouch is the significant
act, and therefore the branchial segments might be expressed in terms
of pouches. In the second ease (B) the formation of a pouch is

THE EVIDENCE OF THE RESPIRATORY APPARATUS 1 67

brought about in consequence of the ingrowth of the mesoblastic
tissues at intervals ; here, although the end-result is the same as in
the first case, the pouch-formation is only secondary, the true
branchial unit is the mesoblastic ingrowth.

The evidence all points directly to the second method of forma-
tion. Thus Shipley, in his description of the development of the
lamprey, says —

"The gill-slits appear to me to be the result of the ventral
downgrowth of mesoblast taking place only at certain places, these
forming the gill-bars. Between each downgrowth the hypoblastic
lining of the alimentary canal remains in contact with the epiblast ;
here the gill-opening subsequently appears about the twenty-second
day."

Dohrn describes and gives excellent pictures of the growth of
the diaphragms, as the Aminoccetes grows in size, pictures which
are distinctly reminiscent of the corresponding illustrations given
by Brauer of the growth of the internal gills in the scorpion embryo.

Another piece of evidence confirmatory of the view that the
branchial segments are really of the nature of internal appendages,
as the result of which gill-pouches are formed, is given by the presence
in each of these branchial bars or diaphragms of a separate ccelomic
cavity. From the walls of this cavity the branchial muscles and
cartilaginous bar are formed.

Now, from an embryological point of view, the vertebrate shows
that it is a segmented animal by the formation of somites, which
consist of a series of divisions of the ccelom, of which the walls form
a series of muscular and skeletal segments. In the head-region, as
already mentioned, such ccelomic divisions form two rows — a dorsal
and a ventral set. From the walls of the dorsal set the somatic
musculature is formed. From those of the ventral set the branchial
musculature. From the latter also the branchial cartilaginous bars
are formed. Thus Shipley, in his description of the development
of the lamprey, says: "The mesoblast between the gills arranges
itself into head-cavities, and the walls of these cavities ultimately
form the skeleton of the gill-arches."

Similarly, in the arthropod, the segments in the embryo are
marked out by a series of co?loniic cavities and Kishinouye has
described in Limulus a separate ccelomic cavity for every one of
the mesosomatic or branchial segments, and he states that in Arachnida

1 68 THE ORIGIN OF VERTEBRATES

the segmental ccelomic cavities extend into the limbs. These
cavities both in the vertebrate and in the arthropod disappear
before the adult condition is reached.

The whole evidence thus points strongly to the conclusion that the
true branchial segmental units are the branchial bars or diaphragms,
not the pouches between them.

It is possible to understand why such prominence has been
given to the conception of the branchial unit as a gill-pouch rather
than as a gill-appendage, when the extraordinary change of appear-
ance in the respiratory chamber of the lamprey which occurs at
transformation, is taken into consideration. This change is of a
very far-reaching character, and consists essentially of the formation
of a new alimentary canal in this region, whereby the pharyngeal
chamber of Ammoccetes is cut off posteriorly from the alimentary
canal, and is confined entirely to respiratory purposes, its original
lumen now forming a tube called the bronchus, which opens into the
mouth and into a series of branchial pouches.

In Fig. 68 I give diagrammatic illustrations taken from Nestler's
paper to show the striking change which takes place at transforma-
tion, (A) representing three branchial segments of Ammoco?tes, and (B)
the corresponding three segments of Petromyzon. The corresponding
parts in the two diagrams are shown by the cartilages (br. cart.), the
sense-organs (S), and the branchial veins ( V. br.) ; the corresponding
diaphragms are marked by the figures 1, 2, 3 respectively. As is
clearly seen, it is perfectly possible in the latter case to describe the
respiratory chamber, as Nestler has done, as divided into a series of
separate smaller chambers — the gill-pouches — by means of a series
of diaphragms or branchial bars. The surface of these gill-pouches
is in part thrown into folds for respiratory purposes, and each gill-
pouch opens, on the one hand, into the bronchus (Bro.), and, on the
other, to the exterior by means of the gill-slit. The branchial unit
in Petromyzon is, therefore, according to Nestler and other mor-
phologists, the folded opposed surfaces of two contiguous diaphragms,
and each one of the diaphragms is intersegmental between two gill-
pouches.

Nestler then goes on to describe the arrangement in Ammoccetes
in the same terms, although there is no bronchus or gill-pouch, but
only an open chamber into which these gill-bearing diaphragms
project, which open chamber serves both for the passage of food and

THE EVIDENCE OF THE RESPIRATORY APPARATUS 1 69

of the water for respiration. This is manifestly the wrong way to
look at the matter: the adult form is derived from the larval, not vice
verm, and the transformation process shows exactly how the gills,
in Rathke's sense, come together to form the bronchus and so make
the gill-pouches of Petromyzon.

When we bear in mind that almost all observers consider that
the internal branchiae of the scorpion group are directly derived

V.br.

-br.cart^/ \

br.cart.

Bro.

Fig. 68. — Diagram of three Branchial Segments of Ammoccetes (A) compared
with three Branchial Segments after Transformation (B) to show how
the Branchial Appendages of Ammoccstes form the Branchial Pouches
of Petromyzon. (After Nestler.)

In both figures the branchial cartilages (br. cart.), the branchial view (V. br.), and the
sense-organs (S), are marked out in order to show corresponding points. The
muscles, blood-spaces, branchial arteries, etc., of each branchial segment are
not distinguished, being represented a uniform black colour. Bro., the bronchus
into which each gill-pouch opens.

from branchial appendages of a kind similar to those of Limulus, it
is evident that a branchial appendage such as that of Ammocoates
might also have arisen from such an appendage, because in various
respects it is easier to compare the branchial appendage of Ammo-
ccetes, than that of the scorpion group, with that of Limulus.

In the case of the scorpions, various suggestions have been made
as to the manner in which such a conversion may have taken place.
The most probable explanation is that given by Macleod, in which

170 THE ORIGIN OF VERTEBRATES

each of the branchiae of the scorpion group is directly compared
with the branchial part of the Limulus appendage which has sunk
into and amalgamated with the ventral surface.

According to this view, the modification which has taken place in
transforming the branchial Limulus-appendage into the branchial
scorpion-appendage is a further stage of the process by which the
Limulus branchial appendage itself has been formed, viz. the getting
rid of the free locomotor segments of the original appendage, thus
confining the appendage more and more to the basal branchial
portion. So far has this process been carried in the scorpion that
all the free part of the appendage has disappeared; apparently, also,
the intrinsic muscles of the appendage have vanished, with the
possible exception of the post-stigmatic muscle, so that any direct
comparison between the branchial appendages of Limulus and the
scorpions is limited to the comparison of their branchiae, their nerves,
and their afferent and efferent blood-vessels.

In the case of Ammoccetes the comparison must be made not
with air-breathing but with water-breathing scorpions, such as
existed in past ages in the forms of Eurypterus, Pterygotus, Slimonia,
and with the crowd of trilobite and Limulus-like forms winch were
in past ages so predominant in the sea ; forms in some of which the
branchial appendages had already become internal, but which, from
the very fact of these forms being water-breathers, probably
resembled, in respect of their respiratory apparatus, Limulus rather
than the present-day scorpion.

On the assumption that the branchial appendages of Ammoccetes,
like the branchial appendages of the scorpion group, are to a certain
extent comparable with those of Limulus, it becomes a matter of great
interest to inquire whether the mode in which respiration is effected
in Ammoccetes resembles most that of Limulus or of the scorpion.

The Origin of the Branchial Musculature.

The difference between the movements of respiration in Limulus
and those of the scorpions consists in the fact that, although in both
cases respiration is effected mainly by dorso-ventral muscles, these
muscles are not homologous in the two cases : in the former, the
dorso-ventral appendage-muscles are mainly concerned, in the latter,
the dorso-ventral somatic muscles.

THE EVIDENCE OF THE RESPIRATORY APPARATUS I 7 I

The paper by Benhani gives a full description of the musculature
of Limulus, and according to his arrangement the muscles are
divided into two sets, longitudinal and dorso-ventral. Of the
latter, each mesosomatic segment possesses a pair of dorso-ventral
muscles, attached to the mid-ventral mesosomatic entochondrite, and
to the tergal surface (Fig. 58, Dv.). These muscles are called by
Benham the vertical mesosomatic muscles. I shall call them the
somatic dorso-ventral muscles, in contradistinction to the dorso-
ventral muscles of the branchial appendages. Of the latter, the two
chief are the external branchial (Fig. 66, m3) "and the posterior
entapophysio-branchial (Fig. 66, m{) ; a third muscle is the anterior
entapophysio-branchial (Fig. 66, m2). Of these muscles, the posterior
entapophysio-branchial (mi) is closely attached along the branchial
cartilaginous bar up to its round-headed termination on the anterior
surface of the appendage. The anterior entapophysio-branchial
muscle (m2) is attached to the branchial cartilage near the
entapophysis.

In the case of the scorpion, as described by Miss Beck, the
branchial appendage has become reduced to the branchiae, and the
intrinsic appendage-muscles have entirely disappeared, with the
possible exception of the small post-stigmatic muscle ; on the other
hand, the dorso-ventral somatic muscles, which are clearly homolo-
gous with the corresponding muscles of Limulus, have remained, and
become the essential respiratory muscles.

Of these two possible types of respiratory movement it is quite
conceivable that in the water-breathing scorpions of olden times
and in their allies, the dorso-ventral muscles of their branchial
appendages may have continued their role of respiratory muscles, and
so have given origin to the respiratory muscles of the ancestors of
Ammoccetes.

The respiratory muscles of Ammoccetes are three in number, and
have been described by Nestler and Miss Alcock as the adductor
muscle, the striated constrictor muscle, and the tubular constrictor
muscle (Fig. 65, m. add,, m.c.s., and m.c.t.). Of these, the constrictor
muscle (Fig. 71, m. con. str.) is in close contact with its cartilaginous
bar, while the adductor (Fig. 71, m. add.) is attached to the cartilage
only at its origin and insertion, and the tubular muscles (Fig. 71,
m. con. tub.) have nothing whatever to do with the cartilage at all,
being attached vent-rally to the connective tissue in the neighbourhood

172 THE ORIGIN OF VERTEBRATES

of the ventral aorta (V.A.), and dorsally to the mid-line between the
dorsal aorta (D.A.) and the notochord.

The close relationship of the constrictor muscle to the carti-
laginous branchial bar does not favour the surmise that this muscle
is homologous with the dorso-ventral somatic muscle of the scorpion.
It is, however, directly in accordance with the view that this muscle
is homologous with one of the dorso-ventral appendage-muscles, such
as the posterior entapophysio-branchial muscle (mi, Fig. 66) of the
Limulus appendage, especially when the homology of the Ammoccetes
branchial cartilage with the Limulus branchial cartilage is borne in
mind. I am, therefore, inclined to look upon the constrictor and
adductor muscles of the Ammoccetes branchial segment as more likely
to have been derived from dorso-ventral muscles which belonged
originally to a branchial appendage, such as we see in Limulus, than
from dorso-ventral somatic muscles, such as the vertical mesosomatic
muscles which are found both in Limulus and scorpion. In other
words, I am inclined to hold the view that the somatic dorso-ventral
muscles have disappeared in this region in Ammoc<etes, while dorso-
ventral appendage-muscles have been retained, i.e. the exact reverse
to what has taken place in the air-breathing scorpion.

I am especially inclined to this view because of the manner in
which it fits in with and explains van Wijhe's results. Ever since
Schneider divided the striated muscles of vertebrates into parietal
and visceral, such a division has received general acceptance and, as
far as the head-region is concerned, has received an explanation in
van Wijhe's work; for Schneider's grouping corresponds exactly to the
two segmentations of the head-mesoblast, discovered by van Wijhe,
i.e. to the somatic and splanchnic striated muscles according to my
nomenclature. Of these two groups the splanchnic or visceral
striated musculature, innervated by the Vth, Vllth, IXth, and Xth
nerves, which ought on this theory to be derived from the muscu-
lature of the corresponding appendages, is, speaking generally, dorso-
ventral in direction in Ammoccetes and of the same character through-
out ; the somatic musculature, on the other hand, is clearly divisible,
in the head region, into two sets — a spinal and a cranial set. The
somatic muscles innervated by the spinal set of nerves, including in
this term the spino-occipital or so-called hypoglossal nerves, are in
Ammoca'tes most sharply defined from all the other muscles of
the body. They form the great dorsal and ventral longitudinal

THE EVIDENCE OF THE RESPIRATORY APPARATUS I 73

body-muscles, which extend dorsally as far forward as the nose and
are developed embryologically quite distinctly from the others, being
formed as muscle-plates (Kastchen). On the other hand, the cranial
somatic muscles are the eye-muscles, the formation of which resembles
that of the visceral muscles, and not of the spinal somatic. Their
direction is not longitudinal, but dorso-ventral ; they cannot, in my
opinion, be referred to the somatic trunk- muscles, and must, therefore,
form a separate group to themselves. Thus the striated musculature
of the Ammoccetes must be divided into (1) the visceral muscles ;
(2) the longitudinal somatic muscles ; and (3) the dorso-ventral somatic
muscles. Of these the 1st, on the view just stated, represent the
original appendage-muscles ; the 2nd belong to the spinal region, and
will be considered with that region ; the 3rd represent the original
segmental dorso-ventral somatic muscles, which are so conspicuous
in the musculature of the Limulus and the scorpion group.

The discussion of this last statement will be given when I come
to deal with the prosomatic segments of Ammoccetes. I wish, here,
simply to point out that van Wijhe has shown that the eye-muscles
develop from his 1st, 2nd, and 3rd dorsal mesoblastic segments, and
therefore represent the somatic muscles belonging to those segments,
while no development of any corresponding muscles takes place in
the 4th, 5th, and 6th segments ; so that if the eye-muscles represent
a group of dorso-ventral somatic muscles, such muscles have been
lost in the 4th, 5th, and 6th segments. The latter segments are,
however, the glossopharyngeal and vagus segments, the branchial
musculature of which is derived from the ventral segments of the
mesoderm. In other words, van Wijhe's observations mean that the
dorso-ventral somatic musculature has been lost in the branchial or
mesosomatic region, while the dorso-ventral appendage musculature
has been retained, and that, therefore, the mode of respiration in
Ammoccetes more closely resembles that of Limulus than of Scorpio.

In addition to these branchial muscles, another and very striking
set of muscles is found in the respiratory region of Ammoccetes — the
so-called tubular muscles. These muscles are of great interest, but
as they are especially connected with the Vllth nerve, their con-
sideration is best postponed to the chapter dealing with that nerve.

Also, in connection with the vagus group of nerves, special sense-
organs are found in the skin covering this mesosomatic region, the
so-called epithelial pit-organs ~(Ep. pit, Fig. 71). They, too, are of

174 THE ORIGIN OF VERTEBRATES

great interest, but their consideration may also better be deferred to
the chapter dealing with those special sense-systems known as the
lateral line and auditory systems.

Comparison of the Branchial Circulation in Ammoccetes and

LlMULUS.

Closely bound up with the respiratory system is the nature of
the circulation of blood through the gills. Before, therefore, proceeding
to the consideration of the segments in front of those which carry
branchire, it is worth while to compare the circulation of the blood
in the gills of Limulus and of Ammoccetes respectively.

In all the higher vertebrates the blood circulates in a closed
system of capillaries, which unite the arterial with the venous systems.
In all the higher invertebrates this capillary system can hardly be
said to exist ; the blood is pumped from the arterial system into blood,
spaces or lacunas, and thus comes into immediate contact with the
tissues. From these it is collected into veins, and so returned to the
heart. There is, in fact, no separate lymph-system in the higher
invertebrates ; the blood-system and lymph-system are not yet
differentiated from each other. This also is the case in Ammoccetes ;
here, too, in many places the blood is poured into a lacunar space,
and collected thence by the venous system ; a capillary system is
only in its commencement and a lymph-system does not yet exist.
In this part of its vascular system Ammoccetes again resembles the
higher invertebrates more than the higher vertebrates.

This resemblance is still more striking when the circulation
in the respiratory organs of the two animals is compared. A
branchial appendage is essentially an appendage whose vascular
system is arranged for the special purpose of aerating blood. In the
.higher vertebrates such a purpose is attained by the pulmonary
capillaries, in Limulus by the division of the posterior surface of the
basal part of the appendage into thin lamellar plates, the interior of
each of which is filled with blood. The two surfaces of each lamella
are kept parallel to each other by means of fibrous or cellular strands
forming little pillars at intervals, called by Macleod " colonettes."
A precisely similar arrangement is found in the scorpion gill-lamella,
as seen in Fig. 69, A, taken from Macleod. In Ammoccetes there are
no well-defined branchial capillaries, but the blood circulates, as in

THE EVIDENCE OE THE RESPIRATORY APPARATUS I 75

the invertebrate gill, in a lamellar space ; here, also, as Nestler has
shown, the opposing walls of the gill-lamella are held in position by
little pillar-like cells, as seen in Fig. 69, B, taken from his paper.

In this representative of the earliest vertebrates the method of
manufacturing an efficient gill out of a lacunar blood-space is pre-
cisely the same as that which existed in Limulus and the scorpion,
and, therefore, as that which existed in the dominant invertebrate
group at the time when vertebrates first appeared. This similarity
indicates a close resemblance between the circulatory systems of the
two groups of animals, and therefore, to the superficial inquirer, would
indicate an homology between the heart of the vertebrate and the
heart of the higher inverte-
brate ; but the former is situ-
ated ventrally to the gut and
the nervous system, while the
latter is composed of a long
vessel which lies in the mid-
dorsal line immediately under
the external dorsal covering.
Indeed, this ventral position of
the heart in the one group of
animals and its dorsal position
in the other, combined with
the corresponding positions of
the central nervous system, is
one of the principal reasons
why all the advocates of the
origin of vertebrates from the

Appendiculata, with the single exception of myself, feel compelled to
reverse the dorsal and ventral surfaces in deriving the vertebrate
from the invertebrate. But there is one most important fact which
ought to make us hesitate before accepting the homology of the
dorsal heart of the arthropod with the ventral heart of the vertebrate
—The heart in all invertebrates is a systemic heart, i.e. drives the
arterial blood to the different organs of the body, and then the veins
carry it back to the respiratory organ, from whence it passes to the

heart.

The only exception to this scheme is found in the vertebrate
where the heart is essentially a branchial heart, the blood being

Fig. 69.— Comparison of Branchial La-
mellae of Limulus and Scorpio with
Branchial Lamellae of Ammoccetes.

A, Branchial lamellae of Scorpio (after
Macleod) ; B, Branchial lamellae of Am-
moccetes (after Nestler).

i76

THE ORIGIN OF VERTEBRATES

driven from the heart to the ventral aorta, from which by the
branchial arteries it is carried to the gills, and then, after aeration, is
collected into the dorsal aorta, whence it is distributed over the
body. The distributing systemic vessel is the dorsal aorta, not the
heart which belongs essentially to the ventral venous system. This
constitutes a very strong reason for believing that the systemic heart
of the invertebrate is not homologous with the heart of the vertebrate.
How, then, did the vertebrate heart arise ?

Let us first see how the blood is supplied to the gills in Limulus.

In Limulus the blood flows into the lamella? from sinuses or
blood-spaces (b.s., Fig. 66) at the base of each of the lamelke, which
sinuses are filled by a vessel which may be called the branchial

Fig. 70. — Longitudinal Diagrammatic Section through the Mesosomatic
Region of Limulus, to show the origin of the Branchial Arteries.
(After Benham.)

L.Y.S., longitudinal venous sinus, or collecting sinus; a.br., branchial arteries-
V.p., veno-pericardial muscles; P., pericardium.

artery, since it is the afferent branchial vessel. On each side of the
middle line of the ventral surface of the body a large longitudinal
venous sinus exists, called by Milne-Edwards the venous collecting
sinus, L. V.S., (Fig. 70 and Fig. 58), which gives off to each of the
branchial appendages on that side a well-defined afferent branchial
vessel — the branchial artery (a. h\). The blood of the branchial artery
flows into the blood-spaces between the anterior and posterior
lamina? of the appendage and thence into the gill-lamella?, from
which it is collected into an efferent vessel or branchial vein, termed
by Milne-Edwards the branchio-cardiac canal, which carries it back
to the dorsal heart. The position of the branchial artery and vein
is shown in Fig. 66, which represents a section through the branchial
appendage of Limulus at right angles to the cartilaginous branchial
bar (br. cart.), just as Fig. 65 represents a section through the

THE EVIDENCE OF THE RESPIRATORY APPARATUS I J J

branchial appendage of Ammoccetes at right angles to the carti-
laginous branchial bar.

Further, the observations of Blanchard, Milne -Ed wards, Kay
Lankester, and Benham concur in showing that in both Limulus and
the scorpion group a striking and most useful connection exists
between the heart and these two collecting venous sinuses, in the
shape of a segmentally arranged series of muscular bands ( V.p., Fig.
70 and Fig. 58), attached, on the one hand, to the pericardium, and
on the other to the venous collecting sinus on each side. These
muscular bands, to which Lankester and Benham have given the
name of ' veno-pericardial muscles,' are so different in appearance
from the rest of the muscular substance, that Milne-Edwards did not
recognize them as muscular, but called them ' brides transparentes.'
Blanchard speaks of them in the scorpion as ' ligaments con-
tractiles,' and considers that they play an important part in assisting
the pulmonary circulation ; for, he says, " en mettant a nu une portion
du cceur, on reinarque que ces battements se font sentir sur les liga-
ments contractiles, et determinent sur les poches pulmonaires une
pression qui fait aussitot refluer et remonter le sang dans les vaisseaux
pneumocardiaques." Lankester, in discussing the veno-pericardial
muscles of Limulus and of the scorpions, says that these muscles
probably contract simultaneously with the heart and are of great
importance in assisting the flow through the pulmonary svstem.
More recently Carlson has investigated the action of these muscles
in the living Limulus and found that they act simultaneously with
the muscles of respiration.

Precisely the same arrangement of veno-pericardial muscles and
of longitudinal venous collecting sinuses occurs in the scorpions. It
is one of the fundamental characters of the group, and we may fairly
assume that a similar arrangement existed in the extinct forms from
which I imagine the vertebrate to have arisen. The further con-
sideration of this group of muscles will be given in Chapter IX.

Passing now to the condition of the branchial blood-vessels of
Ammoccetes, we see that the blood passes into the gill-lamella3 from a
blood-space in the appendage, which can hardly be dignified by the
name of a blood-vessel. This blood-space is supplied by the branchial
artery which arises segmentally from the ventral aorta (V.A.), as seen
in Fig. 71 (taken from Miss Alcock's paper). From the gill-lamellaj
the blood is collected into an efferent or branchial vein (v. br.), which

1 78

THE ORIGIN OF VERTEBRATES

rims, as seeu in Fig. 65, along the free edge of the diaphragm, and
terminates in the dorsal aorta.

The ventral aorta is a single vessel near the heart, but at the com-
mencement of the thyroid it divides into two, and so forms two ventral
longitudinal vessels, from which the branchial arteries arise segmen tally.

''it.lrfrof-W

'TfT-KT.aai-
^^'M-Con- Cut.

-Jyf. Con- Str.

Fig. 71. — Diageam constructed from a series of Transverse Sections through
a Branchial Segment, showing the arrangement and relative positions
of the Cartilage, Muscles, Nerves, and Blood- Vessels.

Nerves coloured red are the motor nerves to the branchial muscles. Nerves coloured
blue are the internal sensory nerves to the diaphragms and the external sensory
nerves to the sense-organs of the lateral line system. Br. cart., branchial
cartilage; M. con. str., striated constrictor muscles; M. con. tub., tubular
constrictor muscles ; M. add., adductor muscle ; D. A., dorsal aorta ; V.A., ventral
aorta; S., sense-organs on diaphragm; n. Lat., lateral line nerve; X., epibran-
chial ganglia of vagus ; B. br. prof. VII., ramus branchialis profundus of facial ;
J. v., jugular vein; Ep. pit., epithelial pit.

From this description it is clear that the vascular supply of the
branchial segment of Ammoccetes would resemble most closely the
vascular supply of the Limulus branchial appendage, if the ventral
aorta of the former was derived from two longitudinal veins, homo-
logous with the paired longitudinal venous sinuses of the latter.

THE EVIDENCE OF THE RESPIRATORY APPARATUS 1 79

A 'priori, such a derivation seems highly improbable ; and yet it
is precisely the manner in which embryology teaches 11s that the
heart and ventral aorta of the vertebrate have arisen.

The Origin of the Invertebrate Heart and the Origin of the

Vertebrate Heart.

Not only does the vertebrate heart differ from that of the inverte-
brate, in that it is branchial while the latter is systemic, but also it
is unique in its mode of formation in the embryo. In the Appen-
diculata the heart is formed as a single organ in the mid-dorsal line
by the growth of the two lateral plates of mesoblast dorsalwards,
the heart being formed where they meet. In Mammalia and Aves,
the heart and ventral aorta commence as a pair of longitudinal veins,
one on each side of the commencing notochord.

If the embryo be removed from the yolk, the surface of the embryo
covering these two venous trunks can be spoken of as the ventral
surface of the embryo at that stage, and indeed we find that in the
present day there is an increasing tendency to speak of this surface
as the ventral surface of the embryo. Thus, Mitsukuri, in his studies
of chelonian embryos, lays great stress on the importance of surface
views and when the embryo has been removed from the yolk,
figures and speaks of its ventral surface. So, also, Locy and ISTeal
find that the best method of seeing the early segments of the embryo
is to remove the embryo from the yolk, and examine what they speak
of as a ventral view. At the period, then, before the formation of the
throat, we may say that on the ventral surface of the embryo a pair
of longitudinal venous sinuses are found, one on each side of the mid-
ventral line, which are in the same position with respect to the mid-
axis of the embryo as are the longitudinal venous sinuses in Limulus.

The next step is the formation of the throat by the extension of
the layers of the embryo laterally to meet in the mid-line and so
form the pharynx, with the consequence that a new ventral surface is
formed ; these two veins, as is well known, travel round also, and,
meeting together in the new mid-ventral line, form the subintestinal
vein, the heart, and the ventral aorta.

What is true of Mammalia and Aves, has been shown by P. Mayer
to be true universally among vertebrates, so that in all cases the heart
and ventral aorta have arisen by the coalescence in the new mid- ventral

i So

THE ORIGIN OF VERTEBRATES

C.N.S

LVS.

line of two longitudinal venous channels, which were originally situ-
ated one on each side of the notochord, in what was then the ventral

surface of this part of the embryo.
This history is especially in-
structive in showing how the
pharyngeal region is formed by
the growing round of the lateral
mesoblast, i.e. the muscular and
other mesoblastic tissues of the
branchial segments, and how the
two longitudinal veins take part
in this process. The phyloge-
netic interpretation of this em-
bryological fact seems to be,
that the new ventral surface of
the vertebrate in this region is
formed, not only by the branchial
appendages, but also by the
growth ventrally of that part
of the original ventral surface
which covered each longitudinal
venous sinus.

The following out of the
consecutive clues, w7hich one
after the other arise in har-
monious succession as the neces-
sary sequence of the original
working hypothesis, brings even now into view the manner in which
the respiratory portion of the alimentary canal arose, and gives
strong hints as to the position of that part of the arthropod which
gave origin to the notochord. Here I will say no more at present,
for the origin of the new alimentary canal of the vertebrate and of
the notochord will be more fittingly discussed as a whole, after all
the other organs of the vertebrate have been compared with the
corresponding organs of the arthropod.

The strong evidence that the vertebrate heart was formed from a
pair of longitudinal venous sinuses on the ventral side of the central
canal, carries with it the conclusion that the original single median
dorsal heart of the arthropod is not represented in the vertebrate,

Fig. 72. — Diagram (Upper Half op
Figure) of the Original Position
of Veins (H) which come together

TO FORM THE HEART OF A VERTEBRATE.

C.N.S., central nervous system; 71c,
notochord; m., myotome.

The lower half of figure shows compara-
tive position of the longitudinal venous
sinus (L.V.S.) in Limulus. C.N.S.,
central nervous system ; A I., alimentary
canal; H., heart ; m., body-muscles.

THE EVIDENCE OF THE RESPIRATORY APPARATUS l8l

for the dorsal aorta cannot by any possibility represent that
heart.

Although it is not now functional the original existence of so
important an organ as a dorsal heart may have left traces of its
former presence ; if so, such traces would be most likely to be visible
in the lowest vertebrates, just as the median eyes are much more
evident in them than in the higher forms. In Fig. 58 the position of
the dorsal heart is shown in Limulus, and in Fig. 70 the shape and
extent of this dorsal heart is shown. It extends slightly into the pro-
somatic region, and thins down to a point there, runs along the length
of the animal and finally thins down to a point at the caudal end.

The heart is surrounded by a pericardium, from which at regular
intervals a number of dorso-ventral muscles pass, to be inserted into
the longitudinal venous sinus on each side. These veno-pericardial
muscles are absolutely segmental with the mesosomatic segments,
and are confined to that region, with the exception of two pairs in the
prosomatic region. Their homologies will be discussed later.

Any trace of a heart such as we have just described must be
sought for in Ammocoetes between the central nervous system and
the mid-line dorsally. Now, in this very position a large striking
mass of tissue is found, represented in section in Fig. 73, /. It
forms a column of similar tissue along the whole mid-dorsal region,
except at the two extremities; it tapers away in the caudal region,
and headwards grows thinner and thinner, so that no trace of it is
seen anterior to the commencement of the branchial region. It
resembles in its dorsal position, in its shape, and in its size a dorsal
heart-tube such as is seen in Limulus and elsewhere, but it differs
from such a tube in its extension headwards. The heart-tube of
Limulus ceases at the anterior end of the mesosomatic region, this
fat- column of Ammoccetes at the posterior end. In its structure there
is not the slightest sign of anything of the nature of a heart ; it is
a solid mass of closely compacted cells, and the cells are all very
full of fat, staining intensely black with osmic acid. Nowhere else
in the whole body of Ammocostes is such a column of fat to be found.
It is not skeletogenous tissue with cells of the nature of cartilage-
cells, as Gegenbaur thought and as Balfour has depicted (Vol. II.,
Fig. 315) in his ' Comparative Embryology,' as though this tissue were
a pari of the vertebral column, but is simply fat-cells, such as might
easily have taken the place of some other previously existing organ.

182

THE ORIGIN OF VERTEBRATES

I do not know how to decide the question which thus arises.
Supposing, for the sake of argument, that this column of fat-cells
has really taken the place of the original dorsal heart, what criterion
would there he as to this ? The heart ex hypothesi having ceased to

function, the muscular tissue
would not remain, and the
space would he filled up,
presumably with some form
of connective tissue. As
likely as not, the connective
tissue might take the form
of fatty tissue, the storage
of fat being a physiological
necessity to an animal, while
at the same time no special
organ has been developed
for such a purpose, but fat
is being laid down in all
manner of places in the
body.

This dorsal fat-column,
as it is seen in Ammoccetes,
is not found in the higher
vertebrates, so that it pos-
sesses, at all events, the
significance of being a pecu-
liarity of ancient times
before the vertebrate skele-
tal column was formed.

I mention it here in
connection with my view

Fig. 73. — Section through the Notochord
(nc), the Spinal Canal and the Fat-
column (/.), of Ammoccetes, drawn from
an Osmic Preparation.

sp. c, spinal cord; gl., glandular tissue filling
the spinal canal; sk., Gegenbaur's skeleto-
genous cells ; p., pigment.

as to the origin of verte-

brates, because there it is,
in the very place where the
dorsal heart ought to have been. For my own part, I should not have
expected that a muscular organ such as the heart would leave any
trace of itself if it disappeared, so that its absence in the dorsal region
of the vertebrate does not seem to me in the slightest degree to
invalidate my theory.

THE EVIDENCE OF THE RESPIRATORY APPARATUS 1 83

Summary.

From the close similarity of structure and position between the branchial
skeleton of Limulus and of Arumocoetes, as given in the preceding chapter, it
logically follows that the branchiae of Ammocoetes must be homologous with the
branchiae of Limulus. But the respiratory apparatus of Limulus consists of
branchial appendages. It follows, therefore, that the branchiae of Ammocoetes.
and consequently of the vertebrates, must have been derived from branchial
appendages, and as they are internal, not external, such branchial appendages
must have been of the nature of ; sunk-in ' branchial appendages. Such
internal appendages are characteristic of the scorpion tribe, and of. perhaps,
the majority of the Palaeostraca, for no external respiratory appendages have
been discovered in any of the sea-scorpions.

In the vertebrates — and it is especially well shown in Ammocoetes — a double
segmentation exists in the head-region, a body or somatic segmentation, and
a branchial or splanchnic segmentation, respectively expressed by the terms
mesomeric and branchiomeric segmentations. The nerves which supply the
latter segments form a very well-marked group (Charles Bell's system of lateral
or respiratory nerves) which do not conform to the system of spinal nerves, for
they do not arise from separate motor and sensory roots, but are mixed nerves
from the very beginning.

The system of cranial segmental nerves is older than the spinal system, and
cannot, therefore, be derived from it, but can be arranged as a system supplying
two segments, somatic and splanchnic, which differ in the following way : Each
somatic segment is supplied by two roots, motor and sensory respectively, as in
the spinal cord segments, while each splanchnic segment possesses only one root,
which is mixed in function.

The peculiarities of the grouping of the cranial segmental nerves, which
have hitherto been unexplained, immediately receive a straightforward and
satisfactory explanation if the splanchnic or branchiomeric segments owe their
origin to a system of appendages after the style of those of Limulus.

In Limulus and all the Arthropoda, the segmentation is double, being com-
posed of (1) somatic or body-segments, constituting the mesomeric segmentation ;
(2) appendage-segments, which, seeing that they carry the branchiae, constitute
a branchiomeric segmentation. Similarly to the cranial region of the vertebrate,
the nerves which supply the somatic segments arise from separate sensory and
motor roots, while the single nerve which supplies each appendage contains all
the fibres for the appendage, both motor and sensory.

It follows from this that the branchial segments supplied by the vagus
and glossopharyngeal nerves ought to have arisen from appendages bearing
branchiae.

Although the evidence of such appendages has entirely disappeared in the
higher vertebrates, together with the disappearance of branchiae, and is not
strikingly apparent in the higher gill-bearing fishes, yet in Ammocoetes, so
great is the difference here from all other fishes, it is natural to describe the
pharyngeal or respiratory chamber as a chamber into which a symmetrical series
of respiratory appendages, the so-called diaphragms, are dependent. Each of
these appendages possesses its own mixed nerve, glossopharyngeal or vagus.

184 THE ORIGIN OF VERTEBRATES

its own cartilage, its own set of visceral muscles, its own sense-organs, just as
do the respiratory appendages of Limulus.

The branchial unit in the vertebrate is not the gill-pouch, but the branchial
bar or appendage between the pouches. Embryology shows how each such
appendage grows inwards, how a ccelomic cavity is formed in it, similarly to the
ingrowing of the branchial appendage in scorpions.

We do not know how the palteostracan sea-scorpions breathed ; they resemble
the scorpion of the present day somewhat in form, but they are in many respects
closely allied to Limulus. The present-day scorpion is a land animal, and the
muscles by which he breathes are dorso-ventral somatic muscles, while those of
Limulus are the appendage-muscles.

The old sea-scorpions very probably used their aj>pendage-muscles after the
Limulus fashion, being water-breathers, even although their respiratory appen-
dages were no longer free but sunk in below the surface of the body. The
probability that such was the case is increased after consideration of the method
of breathing in Ammoccetes, for the respiratory muscles of the latter animal are
directly comparable with the muscles of the respiratory appendages of Limulus,
and are not somatic. Even the gills themselves of Ammoccetes are built up in
the same fashion as are those of Limulus and the scorpions. The conception of
the branchial unit as a gill-bearing appendage, not a gill-pouch, immediately
explains the formation of the vertebrate heart, which is so strikingly different
from that of all invertebrate hearts, in that it originates as a branchial and
not as a systemic heart, and is formed by the coalescence of two long-itudinal
veins.

The origin of these two longitudinal veins is immediately apparent if the
vertebrate arose from a palaeostracan, for in Limulus and the whole scorpion
tribe, in which the heart is a systemic heart, the branchife are supplied with
blood from two large longitudinal venous sinuses, situated on each side of the
middle line of the animal in an exactly corresponding position to that of the two
longitudinal veins, which come together to form the heart and ventral aorta of
the vertebrate. The consideration of the respiratory apparatus and of its blood-
supply in the vertebrate still further points to the origin of vertebrates from the
Palasostraca.

CHAPTEE V

THE EVIDENCE OF THE THYROID GLAND

The value of the appendage-unit in non-branchial segments. — The double nature
of the hyoid segment. — Its branchial part. — Its thyroid part. — The double
nature of the opercular appendage. — Its branchial part. — Its genital part.
— Unique character of the thyroid gland of Ammoccetes — Its structure. —
Its openings. — The nature of the thyroid segment. — The uterus of the
scorpion. — Its glands. — Comparison with the thyroid gland of Ammoccetes.
— Cephalic genital glands of Limulus. — Interpretation of glandular tissue
filling up the brain-case of Ammoccetes. — Function of thyroid gland. —
Relation of thyroid gland to sexual functions. — Summary.

I have now given my reasons why I consider that the glosso-
pharyngeal and vagus nerves were originally the nerves belonging to
a series of mesosoinatic branchial appendages, each of which is still
traceable in the respiratory chamber of Ammoco^tes, and gives the
type-form from which to search for other serially homologous,
although it may be specially modified, segments.

As long as the branchial unit consisted of the gill-pouch the
segments of the head-region were always referred to such units,
hence we find Dohrn and Marshall picturing to themselves the
ancestor of vertebrates as possessing a series of branchial pouches
right up to the anterior end of the body. Marshall speaks of
olfactory organs as branchial sense-organs ; Dohrn of the mouth as
formed by the coalescence of gill- slits, of the trigeminal nerve
as supplying modified branchial segments, etc. ; thus a picture of
an animal is formed such as never lived on this earth, or could be
reasonably imagined to have lived on it. Yet Dohrn's conceptions
of the segmentation were sound, his interpretation only was in
fault, because he was obliged to express his segments in terms of
the gill-pouch unit. Once abandon that point of view and take as
the unit a branchial appendage, then immediately we see that in
the region in front of the branchiae we may still have segments

1 86 THE ORIGIN OF VERTEBRATES

homologous to the branchial segments, originally characterized by
the presence of appendages, but that such appendages need never
have carried branchiae. The new mouth may have been formed by
such appendages, which would express Dohrn's suggestion of its
formation by coalesced gill-slits ; the olfactory organ may have been
the sense-organ belonging to an antennal appendage, which would
be what Marshall really meant in calling it a branchial sense-organ.

The Facial Nerve and the Foremost Eespiratory Segment.

This simple alteration of the branchiomeric unit from a gill-pouch
to an appendage, which may or may not bear branchiae, immedi-
ately sheds a flood of light on the segmentation of the head-region,
and brings to harmony the chaos previously existing. Let us, then,
follow out its further teachings. Next anteriorly to the glosso-
pharyngeal and vagus nerves comes the facial nerve ; a nerve which
supplies the hyoid segment, or, rather, according to van Wijhe the two
hyoid segments, for embryologically there is evidence of two segments.
As already mentioned, the facial nerve is usually included in the
trigeminal or pro-otic group of nerves, the opisthotic group being-
confined to the glossopharyngeal and vagus. This inclusion of the
facial nerve into the pro-otic group of nerves forms one of the main
reasons why this group has been supposed to have originally supplied
gill-pouch segments, for the hyoid segment is clearly associated with
branchiae.

When, however, we examine Ammoco^tes (cf. Figs. 63 and 64)
it is clear that the foremost of the segments forming the respiratory
chamber, which must be classed with the rest of the mesosomatic or
opisthotic segments, is that supplied by the facial nerves.

An examination of this respiratory chamber shows clearly that
there are six pairs of branchial appendages or diaphragms, which are
all exactly similar to each other. These are those already considered,
the foremost of which are supplied by the IXth or glossopharyngeal
nerves. Immediately anterior to this glossopharyngeal segment is
seen in the figures the segment supplied by the Vllth or facial
nerves. It is so much like the segments belonging to the glosso-
pharyngeal and vagus nerves as to make it certain that we are dealing
here with a branchial segment, composed of a pair of branchial
appendages similar to those in the other cases, except that the

Respiratory Append ages
$ Nerve Supply

Tertt.
Tent. in. c.

Hyoiti

6- Br

Fig. 74. — Ventral half
of Head-region of Am-

MOCOiTES.

—-"-Pigment

Somatic muscles coloured
red. Branchial and visce-
ral muscles coloured blue.
Tubular constrictor mus-
cles distinguished from
striated constrictor mus-
cles by simple hatching.
Tent., tentacles ; Tent.m.c,
muco-cartilage of tenta-
cles; TV/, m.c, muco-car-
tilage of the velum ; Hy.
m.c, muco-cartilage of the
hyoid segment; Ps. br.,
pseudo-branchial groove ;
Hr. car/., branchial carti-
lages ; Sp., space between
somatic and splanchnic
muscles ; Th. op., orifice of
thyroid ; //., heart.

1 88 THE ORIGIN OF VERTEBRATES

cartilaginous bar is here replaced by a bar of muco-cartilage and
the branchiae are confined to the posterior part of each appendage.
The anterior portion is, as is seen in Fig. 74, largely occupied by
blood-spaces, but in addition carries the ciliated groove (ps. br.) called
by Dohrn ' pseudo-branchiale Einne.' This groove leads directly
into the thyroid gland, which is a large bilateral organ situated in
the middle line, as seen in Fig. 80 and Fig. 85. As shown by Miss
Alcock, the facial nerve supplies this thyroid gland, as well as the
posterior hyoid branchial segment, and, as pointed out by Dohrn,
there is every reason to consider this thyroid gland as indicative of
a separate segment, especially when van Wijhe's statement that the
hyoid segment is in reality double is taken into account.

The evidence, then, of Ammocoetes points directly to this con-
clusion : The facial nerves represent the foremost of the mesoso-
matic group of nerves, and supply two segments, which have amalga-
mated with each other. The most posterior of these, the hyoid
segment, is a branchial segment of the same character as those
supplied by the vagus and glossopharyngeal nerves ; represents,
therefore, the foremost pair of branchial appendages. The anterior or
thyroid segment, on the other hand, differs from the rest in that,
instead of branchiae, it carries the thyroid gland with its two ciliated
grooves. If this segment, which is the foremost of the mesosomatic
segments, also indicates a pair of appendages which carry the thyroid
gland instead of branchiae, then it follows that this pair of appendages
has joined together in the mid-line ventrally and thus formed a
single median organ — the thyroid gland. If, then, we find that the
foremost of the mesosomatic appendages in the Palaeostraca was really
composed of two pairs of appendages, of which the most posterior
carried branchiae, while the anterior pair had amalgamated in the
mid-line ventrally, and carried some special organ instead of
branchiae, then the accumulation of coincidences is becoming so
strong as to amount to proof of the correctness of our line of
investigation.

The First Mesosomatic Segment in Limulus and its Allies.

What, then, is the nature of the foremost pair of mesosomatic
appendages in Limulus. They differ from the rest of the mesosomatic
appendages in that they do not carry branchiae, and instead of being

THE EVIDENCE OF THE THYROID GLAND

189

separate are joined together in the mid-line ventrally to form a single
terminal plate-like appendage known as the operculum. On its
posterior surface the operculum carries the genital duct on each side.

So also in the scorpion group, the operculum is always found
aud always carries the genital ducts.

A survey of the nature of the opercular appendage demonstrates
the existence of three different types —

1. That of Lirnulus, in which the operculum is free, and carries
only the terminations of the genital ducts. In this type the duct on
each side opens to the exterior separately (Fig. 75).

2. The type of Scorpio, Androctonus, Buthus, etc., in which the

Fig. 75.— Operculum op Limulus to
show the two separate genital
Ducts.

Gen. duct

Fig. 76. — Operculum
Scorpion.

Gen. duct.

of Male

17., terminal chamber, or uterus.

operculum is not free, but forms part of the ventral surface of the
body-wall, but, like Limulus, carries only the terminations of
the genital ducts. In this type the duct on each side terminates
in a common chamber (vagina or uterus), which communicates with
the exterior by a single external median opening. This common
chamber, or uterus ( Ut.), extends the whole breadth of the operculum
(as seen in Fig. 76), and is limited to that segment.

3. The type of Thelyphonus, Hypoctonus, Phrynus, and other
members of the Pedipalpi, in which the operculum forms a part
of the ventral surface of the body wall, but no longer covers only
the termination of the genital apparatus. It really consists of two
parts, a median anterior, which covers the terminal genital apparatus,

i go

THE ORIGIN OF VERTEBRATES

Ut. Masc.

Int. Op.

Ext. Op.

and a lateral posterior, which covers the first pair of gills, or lung-
books, as they are called. In this type (Fig. 77) the genital ducts
terminate in a common chamber or uterus, the nature of which will
be further considered.

As has been pointed out by Blanchard, the terminal genital
organs of the scorpions and the Pedipalpi vary considerably in the
different genera, especially the male genital organs. The general
type of structure is the same, and consists in both male and female
of vasa deferentia, which come together to form a common chamber

before the actual opening
to the exterior. This com-
mon chamber has been
called in the female scor-
pion the vagina, or in
Thelyphonus the uterus.
I shall use the latter term,
in accordance with Tar-
nani's work, and the corre-
sponding chamber in the
male will be the uterus
masculinus.

A considerable discus-
sion has taken place about
the method of action of the
external genital organs in
the members of the scorpion
tribe, into which it is hardly
necessary to enter here.
The evidence points to the
conclusion that in all these forms the operculum covers a median
single chamber or uterus, into which the genital ducts open on each
side, the main channels of emission being provided with a massive
chitinous internal framework. We may feel certain that in the old
extinct sea-scorpions, Eurypterus, etc., a similar arrangement existed,
and that therefore in them also the median portion of the operculum
covered a median chamber or uterus composed of the amalgamation
of the terminations of the two genital ducts, which were originally
separate, as in Limulus.

The observations of Schmidt, Zittel, and others show that the

Fig. 77.— Operculum and Following Seg-
ments of Male Thelyphonus.

Opercular segment is marked out by thick black
line. Ut. Masc, uterus masculinus ; Int. Op.,
internal opening of uterus into genital chamber ;
Ext. Op., common external opening to genital
chamber (Gen. Ch.) and pulmonary chamber.

THE EVIDENCE OF THE THYROID GLAND

I9I

operculum in the old extinct sea-scorpions, Eurypterus, Pterygotus,
etc., belonged to the type of Thelyphonus, rather than to that of
Limulus or Scorpio. In Fig. 78 I give a picture from Schmidt of the
ventral aspect of Eurypterus, and by the side of it a picture of the
isolated operculum. Schmidt considers that there were five branchiae-
bearing segments constituting the mesosoma, the foremost of which
formed the operculum. Such operculum is often found isolated, and
is clearly composed of two lateral
appendages fused together in the
middle line, of such a nature as to
form a median elongated tongue,
which lies between and separates
the first three pairs of branchial
segments. This median tongue,
together with the anterior and
median portion of the operculum,
concealed, in all probability, accord-
ing to Schmidt, the terminal parts
of the genital organs, just as the
median part of the operculum in
Phrynus and Thelyphonus conceals
the complicated terminal portions
of the genital organs. The posterior
part of the operculum, like that of
Phrynus and Thelyphonus, carried
the first pair of branchiae, so Schmidt
thinks from the evidence of markings
on some specimens.

Apparently an opercular ap-
pendage of this kind is in reality
the result of a fusion of the genital
operculum with the first branchial appendage in forms such as the
scorpion; for, in order that the tergal plates may correspond in
number with the sternal in Eurypterus, etc., it is necessary to
consider that the operculum is composed of two sternites joined
together. Similarly in Thelyphonus, Phrynus, etc., this numerical
correspondence is only observed if the operculum is looked upon
as double.

A restoration of the mesosomatic region of Eurypterus, viewed

Fig. 78. — Eurypterus.

The segments and appendages on the
right are numbered in correspon-
dence with the cranial system of
lateral nerve-roots as found in verte-
brates. M., metastoma. The sur-
face ornamentation is represented
on the first segment posterior to the
branchial segments. The opercular
appendage is marked out by dots.

192

THE ORIGIN OF VERTEBRATES

from the internal surface, might be represented by Fig. 79, in which
the thick line represents the outline of the opercular segment, and

the fainter lines the succeeding
branchial segments. The middle
and anterior part of the opercular
segment carried the terminations

Gen. dncfc.

TJt. M&sc.

Gen. duct.

Fig. 79. — Diagram to indicate the

PROBABLE NATURE OF THE MeSOSO-

matic Segments of Eurypterus.

The opercular segment is marked out by
the thick black line. The segments
II. -VI. bear branchiae, and segment I.
is supposed in the male to carry the
uterus masculinus (TJt. Masc.) and
the genital ducts.

of the ' genital

organs

these I

have represented, in accordance
with our knowledge of the nature
of these organs in the present-day
scorpions, as a median elongated
uterus, bilaterally formed, from
which the genital ducts passed,
probably as in Limulus, towards
a mass of generative gland in the
cephalic region, and not as in
Scorpio or Thelyphonus, tailwards
to the abdominal region.

It is possible that in Holm's
representation of Eurypterus, Fig.
104, the genital duct on each side
is indicated.

The Thyroid Gland of Ammoccetes.

If we compare this mesosomatic region of Eurypterus with that
of Ammoccetes, the resemblance is most striking, and gives a mean-
ing to the facial nerve which is in absolute accordance with the
interpretation already given of the glossopharyngeal and vagus
nerves. In both cases the foremost respiratory or mesosomatic
segment is double, the posterior lateral part alone bearing the
branchiae, while the median and anterior part bore in the one animal
the uterus and genital ducts, in the other the thyroid gland and
ciliated grooves. We are driven, therefore, to the conclusion that
this extraordinary and unique organ, the so-called thyroid gland of
Ammoccetes, which exists only in the larval condition and is got rid
of as soon as the adult sexual organs are formed, shows the very form
and position of the uterus of this invertebrate ancestor of Ammo-
ccetes. What, then, is the nature of the thyroid gland in Ammoccetes ?

THE EVIDENCE OF THE THYROID GLAXD

193

Throughout the vertebrate kingdom it is possible to compare the
thyroid gland of one group of animals with that of another without
coming across any very marked difference of structure right down
to and including Petromyzon. When, however, we examine Ammo-
coetes, we find that the thyroid has
suddenly become an organ of much
more complicated structure, covering a
much larger space, and bearing no re-
semblance to the thyroid glands of the
higher forms. At transformation the
thyroid of Animoccetes is largely de-
stroyed, and what remains of the gland
in Petromyzon becomes limited to a few
follicles resembling those of other fishes.
The structure and position of this gland
in Ammoccetes is so well known that it
is unnecessary to describe it in detail.
For the purpose, however, of making
my points clear, I give in Fig. 80 the
position and appearance of the thyroid
gland (Th.) when the skin and under-
lying laminated layer has been re-
moved by the action of hypochlorite of
soda. On the one side the ventral
somatic muscles have been removed to
show the branchial cartilaginous basket-
work.

The series of transverse sections in
Pig. 81 represents the nature of the
organ at different levels in front of and
behind the opening into the respiratory
chamber ; and in Fig. 82 I have sketched
the appearance of the whole gland,
viewed so as to show its opening
into the respiratory chamber and its posterior curled-up termi-
nation.

The series of transverse sections (1-6, Fig. 81) show that we are
dealing here with a central glandular chamber, C (Fig. 81 (6) and
Fig. 82), which opens by the thyroid duct (Th. 0.) into the pharyngeal

Fig. 80. — Ventral View op
Head Region of Ammoccetes.

Th., thyroid gland; M., lower
lip, with its muscles.

i94

THE ORIGIN OF VERTEBRATES

\ Tko

4;-

5 6

Fig. 81. — Samples from a Complete Series of Transverse Sections through

the Thyroid Gland of Ammocoztes.

Sections 1 and 2 are anterior to the thyroid opening, Th. o. ; sections 3, 4, and 5 are
■ through the thyroid opening ; and section 6 is posterior to the thyroid opening
before the commencement of the curled portion.

THE EVIDENCE OF THE THYROID GLAND 1 95

chamber, and is curled upon itself in its more posterior part. This
central chamber divides, anteriorly to the thyroid orifice, into two
portions, A, A' (Fig. 82), giving origin to two tubes, B, B', which lie
close alongside of, and extend further back than, the posterior limit
of the curled portion of the central chamber, C. The structure of
the central chamber, C, and, therefore, of the separate coils, is given
in both Schneider's and Dohrn's pictures, and is represented in
Tig. 81 (6), which shows the peculiar arrangement and character of
the glandular cells typical of this organ, and also the nature of the
central cavity, with the arrangement of the ciliated epithelium. The
structure of each of the lateral tubes, B, is different from that of the
central chamber, in that only half the central chamber is present
in them, as is seen by the comparison of the tube B with the tube C
in Fig. 81 (5 and 6), so that we may look upon the central chamber,
C, as formed of two tubes, similar in structure to the tubes B, which
have come together to form a single chamber by the partial absorp-
tion of their walls, the remains of the wall being still visible as the
septum, which partially divides the chamber, Q, into halves.

In the walls of each of these tubes is situated a continuous
glandular line, the structure of the glandular elements being specially
characterized by the length of the cells, by the large spherical nucleus
situated at the very base of each cell, and by the way in which the
cells form a wedge-shaped group, the thin points of all the wedge-
shaped cells coming together so as to form a continuous line along
the chamber wall. This free termination of the cells of the glaud
in the lumen of the chamber constitutes the whole method for the
secretion of the gland ; there is no duct, no alveolus, nothing but this
free termination of the cells.

Moreover, sections through the portion A, A' (Fig. 82) show that
here, as in the central chamber, C, four of these glandular lines open
into a common chamber, but they are not the same four as in the case
of the central chamber, for if we name these glandular lines on the left
side a b, a V (Fig. 81), and on the right side c d, c' d', then the central
chamber has opening into it the glands a b,c d, while the chambers of
A and A' have opening into them respectively a b, a' V, and c d, c' d'.
Further, the same series of sections shows that the glands a and b are
continuous with the glands a' and b' respectively across the apex of A,
and similarly on the other side, so that the two glandular rows a b
are continuous with the two glandular rows a' //, and we see that the

196

THE ORIGIN OF VERTEBRATES

cavity of the portion A or A' is formed by the bending over of the tube
or horn, B or B', with the partial absorption of the septum so formed
between the tube and its bent-over part. If, then, we uncoil the
curled-up part of C, and separate the portion, B, on each side from the
chamber, C, we see that the so-called thyroid of Ammoccetes may be
represented as in Fig. 83, i.e. it consists of a long, common chamber, C,

Ps br!

Th. o .. -v--: '''

Pit,

•) *

Fig. 82. — Diagbammatic Repbesentation of the so-called Thyboid Gland op

Ammoccetes.

C, central chamber; A, A', anterior extremity; B,B', posterior extremity; Tli.o.,
thyroid opening into respiratory chamber; Ps. br., Ps. br'., ciliated grooves,
Dohrn's pseudo-branchial grooves.

Fig. 83.— Thyboid Gland as it would appeab if the Centbal Chambeb were
Uncueled and the Two Hoens, B, B', sepaeated fbom the Centbal
Chambeb.

which, for reasons apparent afterwards, I will call the palceo-hysteron,
which opens, by means of a large orifice, into the respiratory or
pharyngeal chamber. The anterior end of this chamber terminates in
two tubes, or horns, B, B', the structure of which shows that the median
chamber, C, is the result of the amalgamation of two such tubes, and
consequently in this chamber, or palcco-hystcron, the glandular lines
are symmetrically situated on each side.

Any explanation, then, of the thyroid gland of Ammoccetes, must

THE EVIDENCE OF THE THYROID GLAND 1 97

take into account the clear evidence that it is composed of two
tubes, which have in part fused together to form an elongated central
chamber, in part remain as horns to that chamber, and that in its
walls there exist lines of gland-cells of a striking and characteristic
nature.

Further, this central chamber, with its horns, is not a closed
chamber, but is in communication with the pharyngeal or respiratory
chamber by three ways. In the first place, the central chamber, as
is well known, opens into the respiratory chamber by a funnel-shaped
opening — the so-called thyroid duct (Th. 0.). In the second place,
there exist two ciliated grooves (Ps. br., Ps. br'.), the pseudo-branchial
grooves of Dohrn, which have direct communication with the thyroid
chamber. The manner in which these grooves communicate with the
thyroid chamber has never, to my knowledge, been described pre-
viously to my description in the Journal of Physiology and Anatomy ;
it is very instructive, for, as I have there shown, each groove enters
into the corresponding lateral horn, so that, in reality, there are three
openings into the thyroid chamber or paleeo-hysteron — a median
opening into the central chamber, and a separate opening into each
lateral horn.

The system of ciliated grooves on the inner ventral surface of
the respiratory chamber of Ammoccetes was originally described by
Schneider as consisting of a single median groove, which extends
from the opening of the thyroid to the posterior extremity of the
branchial chamber, and a pair of grooves, or semi-canals, which,
starting from the region of the thyroid orifice, run head wards and
diverge from each other, becoming more and more lateral, and more
and more dorsal, till they come together in the mid-dorsal pharyngeal
line below the auditory capsules. The latter are the pseudo-branchial
grooves of Dohrn, of which I have already spoken. Schneider
looked upon the whole of this system as a single system, for he
speaks of " a ciliated groove, which extends from the orifice of the
stomach {i.e. anterior intestine) to the orifice of the thyroid, then
divides into two, and runs forward right and left of the median ridge,
etc." Dohrn rightly separates the median ciliated groove posterior
to the thyroid orifice (seen in Fig. 81 (6)) from the paired pseudo-
branchial grooves ; the former is a shallow depression which opens
into the rim of the thyroid orifice, while the latter has a much more
intimate connection with the thyroid gland itself.

198 THE ORIGIN OF VERTEBRATES

A series of sections, such as is given in Fig. 81, shows the relation
of this pair of ciliated grooves to the thyroid better than any elaborate
description. In the first place, it is clear that they remain separate
up to their termination — they do not join in the middle line to open
into the thyroid duct ; in the second place, they are separate from
the thyroid orifice — they do not terminate at the rim of the orifice,
as is the case with the median groove just mentioned, but continue
on each side on the wall of the thyroid duct (Fig. 81 (2)), gradually
moving further and further away from the actual opening of the duct
into the pharyngeal chamber. During the whole of their course on
the wall of the funnel-shaped duct they retain the character of
grooves, and are therefore open to the lumen of the duct. The direc-
tion of the groove (Ps. br.) shifts as it passes deeper and deeper
towards the thyroid, until at last, as seen in Fig. 81 (3 and 4), it is
continuous with the narrow diverticulum of the turned-down single
part of the thyroid (B), or turned-down horn, as I have called it.
In other words, the median chamber opens into the pharyngeal or
respiratory chamber by a single large, funnel-shaped opening, and, in
addition, the two ciliated grooves terminate in the lateral horns on
each side, and only indirectly into the central chamber, owing to their
being semi-canals, and not complete canals. If they were originally
canals, and not grooves, then the thyroid of Ammoccetes would be
derived from an organ composed of a large, common glandular
chamber, which opened into the respiratory chamber by means of an
extensive median orifice, and possessed anteriorly two horns, from
each of which a canal or duct passed headwards to terminate some-
where in the region of the auditory capsule.

Dohrn has pointed out that a somewhat similar structure and
topographical arrangement is found in Amphioxus and the Tunicata,
the gland-cells being here arranged along the hypobranchial groove
to form the endostyle and not shut off to form a closed organ, as in
the thyroid of Ammoccetes. Dohrn concludes, in my opinion rightly,
that the endostyle in the Tunicata and in Amphioxus represents the
remnants of the more elaborate organ in Ammoccetes, and that,
therefore, in order to explain the meaning of these organs in the
former animals, we must first find out their meaning in Ammoccetes.
Dohrn, however, goes further than this ; for just as he considers
Amphioxus and the Tunicata to have arisen by degeneration from an
Ammoccetes-like form, so he considers Ammoccetes to have arisen

THE EVIDENCE OF THE THYROID GLAND 1 99

from a degenerated Selachian ; therefore, in order to be logical, he
ought to show that the thyroid of Ammoccetes is an intermediate down-
ward step between the thyroid of Selachians and that of Amphioxus
and the Tunicates. Here, it seems to me, his argument utterly breaks
down ; it is so clear that the thyroid of Petromyzon links on to that
of the higher fishes, and that the Ammocoetes thyroid is so immeasur-
ably more complicated and elaborate a structure than is that of
Petromyzon, as to make it impossible to believe that the Ammoccetes
thyroid has been derived by a process of degeneration from that of
the Selachian. On the contrary, the manner in which it is eaten up
at transformation and absolutely disappears in its original form is,
like the other instances mentioned, strong evidence that we are
dealing here with an ancestral organ, which is confined to the larval
form, and disappears when the change to the higher adult condition
takes place. Dohrn's evidence, then, points strongly to the conclu-
sion that the starting-point of the thyroid gland in the vertebrate
series is to be found in the thyroid of Ammoccetes, which has given
rise, on the one hand, to the endostyle of Amphioxus and the Tuni-
cata, and on the other, to the thyroid gland of Petromyzon and the
rest of the Vertebrata.

The evidence which I have just given of the intimate connection
of the two pseudo-branchial grooves with the thyroid chamber shows,
to my mind, clearly that Dohrn is right in supposing that morpho-
logically these two grooves and the thyroid must be considered
together. His explanation is that the whole system represents a
modified pair of branchial segments distinct from those belonging to
the Vllth and IXth nerves. The cavity of the thyroid and the
pseudo-branchial grooves are, therefore, according to him, the remains
of the gill-pouches of this fused pair of branchial segments, which no
longer open to the surface, and the glandular tissue of the thyroid is
derived from the modified gill-epithelium. This view of Dohrn's,
which he has urged most strongly in various papers, is, I think,
right in so far as the separateness of the thyroid segment is con-
cerned, but is not right, and is not proven, iu so far as concerns the
view that the thyroid gland is a modified pair of gills.

We may distinctly, on my view, look upon the thyroid segment,
with its ciliated grooves and its covering plate of muco-cartilage, as
a distinct paired segment, homologous with the branchial segments,
without any necessity of deriving the thyroid gland from a pair of gills,

200

THE ORIGIN OF VERTEBRATES

The evidence that such a median segment has been interpolated
ventrally between the foremost pairs of branchial segments is
remarkably clear, for the limits ventrally of the branchial segments
are marked out on each side by the ventral border of the carti-
laginous basket-work ; and it is well known, as seen in Fig. 80, that
whereas this cartilaginous framework on the two sides meets together
in the middle ventral line in the posterior branchial region, it diverges
in the anterior region so as to form a tongue-shaped space between

-. IX

. XJ

X2
X3

4-u.LatVII + X

Fig. 84.— Diagram of (A) Ventral Surface and (B) Lateral Surface of Ammo-

C03TES, SHOWING THE ARRANGEMENT OF THE EPITHELIAL PlTS ON THE BRAN-
CHIAL Region, and their innervation by VII., the Facial, IX., the
Glossopharyngeal, and X'-X", the Vagus Nerves.

the branchial segments on the two sides. This space is covered over
with a plate of muco-cartilage which bears on its inner surface the
thyroid gland.

In addition to this evidence that we are dealing here with a
ventral tongue-like segment belonging to the facial nerve which is
interpolated between the foremost branchial segments, we find the
most striking fact that at transformation the whole of this muco-
cartilaginous plate disappears, the remarkable thyroid gland of the

THE EVIDENCE OF THE THYROID GLAND

20I

Ammocoetes is eaten up, and nothing is left except a small, totally
different glandular mass ; and now the cartilaginous basket-work
meets together in the middle line in this region as well as in the
more posterior region. In other words, the striking characteristic

\— -v

Ps.br

£&**'

8 X,

9 X6

Fig. 85. — Facial Segment op Ammoccetes maeked out by Shading.

VII. 1, thyroid part of segment ; VII. 2, hyoid or branchial part ; 3-9, succeeding
branchial segments belonging to IXth and Xth nerves ; V, the velar folds ;
Ps. br., Dohrn's pseudo-branchial groove; Th. o., thyroid opening; C, curled
portion of thyroid.

of transformation here is the destruction of this interpolated seg-
ment, and the resulting necessary drawing together ventrally of
the branchial segments on each side.

Moreover, another most instructive piece of evidence pointing in
the same direction is afforded by the behaviour of the ventral epithelial

202 THE O RIG IX OF VERTEBRATES

pits, as determined by Miss Alcock. Although there is no indication
on the ventral surface of the skin of any difference between the
anterior and posterior portions of the respiratory region, yet when
the ventral rows of the epithelial pits supplied by each branchial
nerve are mapped out, we see how the most anterior ones diverge
more and more from the mid-ventral line, following out exactly the
limits of the underlying muco-cartilaginous thyroid plate (Fig. 84).

The whole evidence strongly leads to the conclusion that the
thyroid portion of the facial segment was inserted as a median tongue
between the foremost branchial segments on each side, and that,
therefore, the whole facial segment, consisting as it does of a thyroid
part and a hyoid or branchial part, may be represented as in Fig.
85, which is obtained by splitting an Ammoccetes longitudinally
along the mid-dorsal line, so as to open out the pharyngeal chamber
and expose the whole internal surface. The facial segment is marked
out by shading lines, the glosso-pharyngeal and vagus segments and
the last of the trigeminal segments being indicated faintly. The
position of the thyroid gland is indicated by oblique lines, C being
the curled portion.

The Uterus of the Scorpion Group.

Seeing how striking is the arrangement and the structure of the
glandular tissue of this thyroid, how large the organ is and how
absolutely it is confined to Ammoccetes, disappearing entirely as
such at transformation, we may feel perfectly certain that a corre-
sponding, probably .very similar, organ existed in the invertebrate
ancestor of the vertebrate ; for the transformation process consists
essentially of the discarding of invertebrate characteristics and the
putting on of more vertebrate characters ; also, so elaborate an organ
cannot possibly have been evolved as a larval adaptation during the
life of Ammoccetes. We may therefore assert with considerable con-
fidence that the thyroid gland was the iKiloco-liysieroiii, and was
derived from the uterus of the ancient pala^ostracan forms. If, then,
it be found that a glandular organ of this very peculiar structure and
arrangement is characteristic of the uterus of any living member of
the scorpion group, then the confidence of this assertion is greatly
increased.

In Limulus, as already stated, the genital ducts open separately

THE EVIDENCE OF THE THYROID GLAND

203

ou each side of the operculum, and do not combine to form a
uterus ; I have examined them and was unable to find any glandular
structure at all resembling that of the thyroid gland of Ammoccetes.
I then turned my attention to the organs of the scorpion, in which
the two ducts have fused to form a single uterus.

I there found that both in the male and in the female the genital

Fig. 86. — Section through the Terminal Chamber or Uterus of the Male

Scorpion.

C, cavity of chamber. A portion of the epithelial lining of the channels of emission
is drawn above the section of the uterus.

ducts on each side terminate in a common chamber or uterus, which
underlies the whole length of the operculum, and opens to the
exterior in the middle line, as shown in Fig. 76. In transverse
section, this uterus has the appearance shown in Fig. 86, i.e. it is
a large tube, evidently expansible, lined with a chitinous layer and
epithelial cells belonging to the chitinogenous layer, except in two
symmetrical places, where the uniformity of the uterine wall is

204

THE ORIGIN OF VERTEBRATES

interrupted by two large, remarkable glandular structures. The
structure of these glands is better shown by means of sagittal sec-
tions. They are composed of very long, wedge-shaped cells, each of
which possesses a large, round nucleus at the basal end of the cell
(Pig. 87). These cells are arranged in bundles of about eight to ten,
which are separated from each other by connective tissue, the apex
of each conical bundle being directed into the cavity of the uterus ;
where this brush -like termination of the cells reaches the surface, the
chitinous layer is absent, so that this layer is, on surface view, seen

Fig. 87. — Longitudinal Sec-
tion THROUGH THREE OF

the Cones op the Uterine
Glands op the Scorpion.

Fig. 88. — Sagittal Section through
the Uterine Gland of Scorpion,
showing the internal chitinous
Surface (b) and the Glandular
Cones (a) cut through at various

DISTANCES FROM THE INTERNAL SUR-
FACE.

(Fig. 88 (b)) to be pitted with round holes over that part of the
internal surface of the uterus where these glands are situated. Each
of these holes represents the termination of one of these cone-shaped
wedges of cells. If the section is cut across at right angles to the
axis of these cones, then its appearance is represented in Fig. 88 («),
and shows well the arrangement of the blocks of cells, separated from
each other by connective tissue. When the section passes through
the basal part of the cones, and only in that case, then the nuclei
of the cells appear, often in considerable numbers in one section, as

THE EVIDENCE OF THE THYROID GLAXD

205

is seen in Yv* 89. In Fig. 88 the section shows at b the holes in
the chitin in which the cones terminate, and then a series of layers
of sections through the cones further and
further away from their apices.

These conical groups of long cells, repre-
sented in Fig. 87, form on each side of the
uterus a gland, which is continuous along
its whole length, and thus forms a line
of secreting surface on each side, just as
in the corresponding arrangement of the
glandular structures in the thyroid of Am-
moccetes. This uterus and glandular ar-
rangement is found in both sexes ; the gland is, however, more
developed in the male than in the female scorpion.

The resemblance between the structure of the thyroid of Ammo-
ccetes and the uterus of the scorpion is most striking, except in two
respects, viz. the nature of the lining of the non-glandular part of
the cavity — in the one case ciliated, in the other chitinous — and the
place of exit of the cavity, the thyroid of Ammoccetes opening into

Fig. 89. — Transverse Sec-
tion THROUGH THE BASAL

Part of the Uterine
Glands op the Scorpion.

AMMOCCETES.

SCORPION.

Muco-cartilage Operculum

Branchial cartilage
Fig. 90.— Section op Central Chamber op Thyroid op Ammoccetes and Section

of Uterus of Scorpion.

the respiratory chamber, while the uterus of Scorpio opens direct to
the exterior.

With respect to the first difference, the same difficulty is met

206 THE ORIGIN OF VERTEBRATES

with in the comparison of the ciliated lining of the tube in the
central nervous system of vertebrates with the chitinous lining
of the intestine in the arthropod. Such a difference does not seem
to me either unlikely or unreasonable, seeing that cilia are found
instead of chitin in the intestine of the primitive arthropod Peri-
patus. Also the worm- like ancestors of the arthropods almost
certainly possessed a ciliated intestine. Finally, the researches of
Hardy and McDougall on the intestine of Daphnia point directly to
the presence of a ciliated rather than a chitinous epithelial lining of
the intestine in this animal — all evidence pointing to the probability
that in the ancient arthropod forms, derived as they were from the
annelids, the intestine was originally ciliated and not chitinous. It
is from such forms that I suppose vertebrates to have sprung, and
not from forms like the living king-crabs, scorpions, Apus, Bran-
chipus, etc. I only use them as illustrations, because they are the
only living representatives of the great archaic group, from which
the Crustacea, Arachnida, and Vertebrata all took origin.

The second difference is more important, and is at first sight
fatal to any comparison between the two organs. How is it possible
to compare the uterus of the scorpion, which opens on the surface by
an external genital opening, with the thyroid of Amnioccetes, which
opens by an internal opening into the respiratory chamber ? However
close may be the histological resemblance of structure in the two
cases, surely such a difference is too great to be accounted for.

It is, however, to be remembered that the operculum of Scorpio
covers only the terminal genital apparatus, and does not, therefore,
resemble the operculum of the presumed ancestor of Ammoccetes,
which, as already argued, must have resembled the operculum of
Thelyphonus with its conjoint branchial and genital apparatus,
rather than that of Scorpio. Before, therefore, making too sure of
the insuperable character of this difficulty, we must examine the
uterus of the Pedipalpi, and see the nature of its opening.

The nature of the terminal genital organs in Thelyphonus has
been described to some extent by Blanchard, and more recently by
Tarnani. The ducts of the generative organs terminate, according to
the latter observer, in the large uterus, which is found both in the
male and female ; he describes the walls of the uterus in the female
as formed of elongated glandular epithelium, with a strongly-
developed porous, chitinized intima. In the male, he says that the

THE EVIDENCE OF THE THYROID GLAND

207

epithelium of the uterus masculiuus and its processes is extraordi-
narily elongated, the chitin covering being thick. In these animals,
then, the common chamber or uterus into which the genital ducts
empty, which, like the corresponding chamber in the scorpion,
occupies the middle region of the operculum, is a large and con-
spicuous organ. Further, and this is a most striking fact, the
uterus masculinus does not open direct to the exterior, but into the
genital cavity, " which lies above the uterus, so that the latter is
situated between the lower wall of the genital cavity and the outer
integument." The opening,
therefore, of the uterus is not
external but internal, into the ^ A

large internal space known
as the genital cavity. The
arrangement is shown in Fig.
91, taken from Tarnani's
paper, which represents a
diagrammatic sagittal section
through the exit of the male
genital duct. Yet another
most striking fact is described
by Tarnani. This genital
cavity is continuous with the

Gen . Ch.

Ut.Masc.

— I -II

Ut.Masc.

1 - II

WJ— -Int. Op
--Eart.Op.

.---Ill

Fig. 91. — Sagittal Median Diagrammatic
Section through the Operculum of the
Male Thelyphonus. (From Tarnani.)

pulmonary or gill cavities on The thick line is the operoulum> composed of

each side, SO that instead of a two segments, I. and II. Ut. Masc, uterus
single opening for the genital masculinus ; Gen. Ch., genital chamber ; Int.
0 L ° Op., internal opening ; Ext. Op., external

products and one on each Side opening common to the genital and respira-
for each gill-pouch, as would tory organs,
be the case if the arrangement

was of the same kind as in the scorpion, there is a single large
chamber, the genital chamber, common to both respiratory and
genital organs.

This genital chamber, according to Tarnani, opens to the exterior
by a single median opening between the operculum and the succeed-
ing segment ; similarly, a communication from side to side exists
between the second pair of gill-pouches. I have been able to
examine Hypoetonm formosus and Thelyphonits caudatus, and in both
cases, in both male and female, the opening to the exterior of the
common chamber for respiration and for the genital products was

208 THE ORIGIN OF VERTEBRATES

not a single opening, as described by Tarnani in Thelyphonus aspe-
ratus, but on each side of the middle line, a round orifice closed by a
lid, like the nest of the trapdoor spider, led into the common genital
chamber (Gen. Ch.) into which both uterus and gills opened. In
Fig. 77 I have endeavoured to represent the arrangement of the
genital and respiratory organs in the male Thelyphonus according to
Tarnani's and my own observations.

If we may take Thelyphonus as a sample of the arrangement in those
scorpions in which the operculum was fused with the first branchial
appendage, among which must be included the old sea-scorpions, then
it is most significant that their uterus should open internally into a
cavity which was continuous with the respiratory cavity. Thus not
only the structure of the gland, but also the arrangement of the internal
opening into the respiratory, or, as it became later, the pharyngeal
cavity, is in accordance with the suggestion that the thyroid of Ammo-
ccetes represents the uterus of the extinct Eurypterus-like ancestor.

Into this uterus the products of the generative organs were poured
by means of the vasa deferentia, so that there was not a single
median opening or duct in connection with it, but also two side
openings, the terminations of the vasa deferentia. These are described
by Tarnani in Thelyphonus as opening into the two horns of the
uterus, which thus shows its bilateral character, although the body
of the organ is median and single ; these ducts then pass within the
body of the animal, dorsal to the uterus, towards the testes or ovaries
as the case may be, organs which are situated in these animals, as in
other scorpions, in the abdomen, so that the direction of the ducts
from the generative glands to the uterus is headwards. If, however,
we examine the condition of affairs in Limulus, we find that the
main mass of the generative material is cephalic, forming with the
liver that dense glandular mass which is packed round the supra -
(esophageal and prosomatic ganglia, and round the stomach and
muscles of the head-region. From this cephalic region the duct
passes out on each side at the junction of the prosomatic and nieso-
somatic carapace to open separately on the posterior surface of the
operculum, near the middle line, as is indicated in Fig. 75.

We have, therefore, two distinct possible positions for the genital
ducts among the group of extinct scorpion-like animals, the one
from the cephalic region to the operculum, and the other from the
abdominal region to the operculum.

THE EVIDENCE OF THE THYROID GLAND 209

The Generative Glands of Limulus and its Allies.

The whole argument, so far, has in every case ended with the
conclusion that the original scorpion-like form with which I have
been comparing Aminoccetes resembled in many respects Limulus
rather than the present-day scorpions, and therefore in the case also
of the generative organs, with which the thyroid gland or palteo-
hysteron was in connection, it is more probable that they were
cephalic in position rather than abdominal. If this were so, then
the duct on each side, starting from the median ventral uterus, would
take a lateral and dorsal course to reach the huge mass of generative
gland lying within the prosomatic carapace, just as I have repre-
sented in the figure of Eurypterus (Fig. 79), a course which would
take much the same direction as the ciliated groove in Ammocuetes.

We ought, therefore, on this supposition, to expect to find the
remains of the invertebrate generative tissue, the ducts of which
terminated in the thyroid, in the head-region, and not in the
abdomen.

Upon removal of the prosomatic carapace of Limulus, a large
brownish glandular-looking mass is seen, in which, if it happens to
be a female, masses of ova are very conspicuous. This mass is com-
posed of two separate glands, the generative glands and the hepatico-
pancreatic glands — the so-called liver — and surrounds closely the
central nervous system and the alimentary canal. From the genera-
tive glands proceed the genital ducts to terminate on the posterior
surface of the operculum. From the liver ducts pass to the pyloric
end of the cephalic stomach, and carry the fluid by means of which
the food is digested, for, in all these animals, the active digesting
juices are formed in the so-called liver, and not in the cells of the
stomach or intestine.

It is a very striking fact that the brain of Ammoccotes is much
too small for the brain-case, and that the space between brain and
brain-case is filled up with a very peculiar glandular-looking tissue,
which is found in Ammocoetes and not elsewhere. Further, it is also
striking that in the brain of Ammocoetes there should still exist the
remains of a tube extending from the IVth ventricle to the surface at
the conns post-eommismralis, which can actually be traced right into
this tissue on the outside of the brain (see Fig. 13, a-e, PI. XXVI.,
in my paper in the Quarterly Journal of Microscopical Science).

2IO

THE ORIGIN OF VERTEBRATES

This, in my opinion, is the last remnant of one of the old liver-ducts
which extended from the original stomach and intestine into the
cephalic liver-mass. This glandular-looking material is shown
surrounding the pineal eye and its nerve, in Fig. 31, also in
Fig. 22, and separately in Fig. 92. It is composed of large cells,
with a badly staining nucleus, closely packed together with lines
of pigment here and there between the cells ; this pigment is
especially congregated at the spot where the so-called liver-duct
loses itself in this tissue. The protoplasm in these large cells does
not stain well, and with osmic acid gives no sign of fat, so that

Ahlborn's description of this tissue as a
peculiar arachnoideal fat -tissue is not
true ; peculiar it certainly is, but fatty
it is not.

/:.. '"l^^V ^n*s kissue nas Deen largely de-

: 7\ scribed as a peculiar kind of connective

tissue, which is there as packing mate-
rial, for the purpose of steadying a brain
too small for its case. On the face of
it such an explanation is unscientific ;
certainly for all those who really believe
in evolution, it is out of the question
to suppose that a brain-case has been
laid down in the first instance too large for the brain, in order
to provide room for a subsequent increase of brain ; just as it is
out of the question to suppose that the nervous system was laid
down originally as an epithelial tube in order to provide for the
further development of the nervous system by the conversion of
more and more of that tube into nervous matter. Yet this latter
proposition has been seriously put forward by professed believers in
evolution and in natural selection.

This tissue bears no resemblance whatever to any form of con-
nective tissue, either fatty or otherwise. By every test this tissue
tells as plainly as possible that it is a vestige of some former
organ, presumably glandular, which existed in that position ; that
it is not there as packing material because the brain happened
to be too small for its case, but that, on the contrary, the brain
is too small for its case, because the case, when it was formed,
included this organ as well as the brain ; in other words, this tissue

Fig. 92. — Drawing of the
Tissue which surrounds
the Brain op Ammocoetes.

THE EVIDENCE OF THE THYROID GLAND 211

is there because it is the remnant of the great glandular mass which
so closely surrounds the brain and alimentary canal in animals such
as Limulus. In my paper in the Quarterly Journal of Microscopical
Science, in which I was comparing the tube of the vertebrate nervous
system with the alimentary canal of the invertebrate, I spoke of this
tissue as being the remnant of the invertebrate liver. At the same
time the whole point of my argument was that the glandular material
surrounding the brain of Limulus was made up of two glands — liver
and generative gland — so that this tissue might be the remnant of
either one or the other, or both. All I desired, at that time, was
to point out the glandular appearance of this so-called packing tissue,
which surrounded the brain-region of Ammoccetes, in connection with
the fact that the brain and alimentary canal of Limulus were closely
surrounded with a glandular mass composed partly of liver, partly of
the generative gland. At present, I think these large cells found
round the brain in Ammoccetes are much more likely to be the
remnant of the generative gland than of the liver ; the size of the
cells and their arrangement recalls Owen's picture of the generative
gland in Limulus, and seeing how important all generative glands
are in their capacity of internal secreting glands, apart entirely from
the extrusion of the ripe generative products, and how unimportant
is an hepato-pancreas when the alimentary canal is closed, it is much
more likely that of the two glands the former would persist longer
than the latter. It may be that all that is left of the old hepato-
pancreas consists of the pigment so markedly found in between these
cells, especially at the place where the old liver-duct reaches the
surface of the brain ; just as the only remnant of the two pineal eyes
in the higher vertebrates is the remains of the pigment, known as
brain-sand, which still exists in the pineal gland of even the highest
vertebrate. This, however, is a mere speculation of no importance.
What is important is the recognition of this tissue round the brain
as the remnant of the glandular mass round the brain of animals such
as Limulus. Still further confirmation of the truth of this comparison
will be given when the origin of the auditory organ comes up for
discussion.

I conclude, therefore, from the evidence of Ammoccetes, that the
generative glands in the ancestral form were situated largely in the
cephalic region, and suggest that the course and direction of the ciliated
pseudo-branchial grooves on each side indicate the direction of the

2 12 THE ORIGIN OF VERTEBRATES

original opercular ducts by which the generative products were con-
veyed to the uterine chamber, i.e. to the chamber of the thyroid
gland, and thence to the common genital and respiratory cavity, and
so to the exterior.

It is easy to picture the sequence of events. First, the generative
glands, chiefly confined to the cephalic region, communicating with
the exterior by separate ducts on the inner surface of the operculum
as in Limulus. Then, in connection with the viviparous habit, these
two oviducts fused together to form a single chamber, covered by the
operculum, which opened out to the exterior by a single opening as
in Scorpio : or, in forms such as Eurypterus, in which the operculum
had amalgamated with the first branchial appendage and possessed a
long, tongue-like ventral projection, the amalgamated ducts formed
a long uterine chamber which opened internally into the genital
chamber — a chamber which, as in Thelyphonus, was common with
that of the two gill-chambers, while at the same time the genital ducts
from the cephalic generative material opened into two uterine horns
which arose from the anterior part of the uterus, as in Thelyphonus.

Such an arrangement would lead directly to the condition found
in Ammoccetes, if the generative material around the brain lost its
function, owing to a new exit for generative products being formed
in the posterior part of the body. The connection of the genital duct
with this cephalic gland being then closed and cut off by the brain-
case, the position of the oviducts would still be shown by the ciliated
grooves opening into the folded-down thyroid tube, i.e. the folded-
down horns of the uterus ; the uterus itself would remain as the
main body of the thyroid and still open by a conspicuous orifice into
the common respiratory chamber. Next, in the degeneration process,
we may suppose that not only the oviducts opened out to form the
ciliated groove, but that the uterine chamber itself also opened out,
and thus formed the endostyle of Amphioxus and of the Tunicata.

It might seem at first sight improbable that a closed tube should
become an open groove, although the reverse phenomenon is common
enough ; the difficulty, however, is clearly not considered great, for it
is precisely what Dohrn imagines to have taken place in the conversion
of the thyroid of Ammoco^tes into the endostyle of Amphioxus and
the Tunicata ; it is only carrying on the same idea a stage further to
see in the open, ciliated groove of Ammocectes the remains of the
closed genital duct of Limulus and its allies.

THE EVIDENCE OF THE THYROID GLAND 213

Such is the conclusion to which the study of the thyroid gland
in Ammoccetes seems to me to lead, and one cannot help wondering
why such an unused and rudimentary organ should have remained
after its original function had gone. Is it possible to find out its
function in Ammocretes ?

The Function of the Thyroid Gland in Ammoccetes.

The thyroid gland has been supposed to secrete mucus into the
respiratory chamber for the purpose of entangling the particles of food,
and so aiding in digestion. I see no sign of any such function ; neither
by the thionin method, nor by any other test, have Miss Alcock and
myself ever been able to see any trace of mucous secretion in the thy-
roid, and, indeed, the thyroid duct is always remarkably free from any
sign of any secretion whatever. Not only is there no evidence of any
mucous secretion in the thyroid of the fully developed Ammoccetes,
but also no necessity for such secretion from Dohrn's point of view,
for so copious a supply of mucus is poured out by the glands of the
branchiae, along- the whole pharyngeal tract, especially from the cells
of the foremost or hyoid gills, as to mix up with the food as
thoroughly as can possibly be needed. Further, too, the ciliated
pharyngeal bands described by Schneider are amply sufficient to
move this mixed mass along in the way recpiired by Dohrn. Finally,
the evidence given by Miss Alcock is absolutely against the view that
the thyroid takes any part in the process of digestion, while, on the
other hand, her evidence directly favours the view that these
glandular branchial mucus-secreting cells play a most important part
in the digestive process.

In Fig. 93, A is a representation of the respiratory tissue of a
normal gill ; B is the corresponding portion of the first or hyoid gill,
in which, as is seen, the whole of the respiratory epithelium is
converted into gland-tissue of the nature of mucous cells.

To sum up, the evidence is clear and conclusive that the Ammo-
cartes possesses in its pharyngeal chamber mucus-secreting glands,
which take an active part in the digestive process, which do not in
the least resemble either in structure or arrangement the remarkable
cells of the thyroid gland, and that the experimental evidence that
the latter cells either secrete mucus or take any part in digestion
is so far absolutely negative. It is, of course, possible, that they

214

THE ORIGIN OF VERTEBRATES

may contain mucin in the younger developmental stages, and there-
fore possible that they might at that stage secrete it ; they certainly,
however, show no sign of doing so in their more adult condition, and
cannot be compared in the very faintest degree to the glandular cells
of the pharyngeal region. It is also perfectly possible for gland-cells
belonging to a retrograde organ to become mucus-secreting, and so to

give rise to the cells of Am-
phioxus and the Tunicata.

If, then, these cells were
not retained for digestive
purposes, what was their
function ? To answer this
question we must first know
the function of the corre-
sponding gland-cells in the
uterus of the scorpion, which
undoubtedly secreted into
the cavity of the uterus and
took some part in connection
with the generative act, and
certainly not with digestion.
What the function of these
cells is or in what way they
act I am unable at present
to say. I can only suppose
that the reason why the
thyroid gland has persisted
throughout the vertebrate
kingdom, after the genera-
tive tissues had found a new
outlet for their products in the body-cavity of the posterior region,
is because it possessed some important function in addition to that
connected with the exit of the products of the generative organs ; a
function which was essential to the well-being, or even to the life of
the animal. We do not know its function in the scorpion, or the
nature of its secretion in that animal. We know only that physiology
at the present day has demonstrated clearly that the actual external
secretion of a gland may be by no means its most important function ;
in addition, glands possess what is called an internal secretion, viz. a

Fig. 93.— A, Portion op a Gill op Ammo-

C03TES WITH ORDINARY RESPIRATORY EPI-
THELIUM ; B, Corresponding Portion of
the First or Hyoid Gill.

THE EVIDENCE OF THE THYROID GLAND 215

secretion into the blood and lymph, and this latter secretion may be
of the most vital importance. Now, the striking fact forces itself
prominently forward, that the thyroid gland of the higher vertebrates
is the most conspicuous example of the importance of such internal
secretion. Here, although ductless, we have a gland which cannot
be removed without fatal consequences. Here, in the importance of
its internal secretion, we have a reason for the continued existence
of this organ ; an organ which remains much the same throughout
the Yertebrata down to and including Petromyzon, but, as is seen
at transformation, is all that remains of the more elaborate, more
extensive organ of Ammoccetes. Surely we may argue that it is
this second function which has led to the persistence of the thyroid,
and that its original form, without its original function, is seen in
Ammoccetes, because that is a larval form, and not a fully-developed
animal. As soon as the generative organs of Petromyzon are developed
at transformation, all trace of its connection with a genital duct
vanishes, and presumably its internal secretory function alone remains.

Yet, strange to say, a mysterious connection continues to exist
between the thyroid gland and the generative organs, even up to
the highest vertebrate. That the thyroid gland, situated as it is
in the neck, should have any sympathy with sexual functions if it
was originally a gland concerned with digestion is, to say the least
of it, extremely unlikely, but, on the contrary, likely enough if it
originated from a glandular organ in connection with the sexual
organs of the palasostracan ancestor of the vertebrate.

Freund has shown, and shown conclusively, that there is an
intimate connection between the condition of the thyroid gland and
the state of the sexual organs, not only in human beings, but also
in numerous animals, such as dogs, sheep, goats, pigs, and deer. He
points out that the swelling of the gland, which occurs in consequence
of sexual excitement (a fact mentioned both in folk-lore tales and in
poetical literature), and also the swelling at the time of puberty, may
both lead to a true goitrous enlargement ; that most of the permanent
goitres commence during a menstrual period ; that during pregnancy
swelling of the thyroid is almost universal, and may become so ex-
treme as to threaten suffocation, or even cause death ; that the period
of puberty and the climacteric period are the two maximal periods
for the onset of goitre, and that exophthalmic goitre especially is
associated with a special disease connected with the uterus.

2l6 THE ORIGIN OF VERTEBRATES

Summary.

Step by step in the preceding chapters the evidence is accumulating- in
favour of the origin of vertebrates from a member of the palseostracan group.
In a continuously complete and harmonious manner the evidence has throughout
been most convincing when the vertebrate chosen for the purpose of my argu-
ments has been Ammoccetes.

So many fixed points have been firmly established as to enable us to proceed
further with very great confidence, in the full expectation of being able
ultimately to homologize the Vertebrata with the Palfleostraca even to minute
details.

Perhaps the most striking and unexpected result of such a comparison is the
discovery that the thyroid gland is derived from the uterus of the palfeostracan
ancestor. Yet so clear is the evidence that it is difficult to see how the homology
can be denied.

In the one animal (Palasostraca) the foremost pair of mesosomatic appendages
forms the operculum, which always bears the terminal generative organs and
is fused in the middle line. In many forms, essentially in Eurypterus and the
ancient sea-scorpions, the operculum was composed of two segments fused
together : an anterior one which carried the uterus, and a posterior one which
carried the first pair of branchiae.

In the other animal (Ammoccetes) the foremost segments of the mesosomatic
or respiratory region, immediately in front of the glossopharyngeal segments,
are supplied by the facial nerve, and are markedly different from those supplied
by the vagus and glossopharyngeal, for the facial supplies two segments fused
together ; the anterior one, the thyroid segment, carrying the thyroid gland,
the posterior one, the hyoid segment, carrying the first pair of branchiae.

Just as in Eurypterus the fused segment, carrying- the uterus on its internal
surface, forms a long- median tongue which separates the most anterior branchial
segments on each side, so also the fused segment carrying the thyroid forms in
Ammoccetes a long median tongue, which separates the most anterior branchial
segments on each side.

Finally, and this is the most conclusive evidence of all, this thyroid gland
of Ammocoetes is totally unlike that of any of the higher vertebrates, and.
indeed, of the adult form Petromyzon itself, but it forms an elaborate com-
plicated organ, which is directly comparable with the uterus and genital ducts
of animals such as scorpions. Not only is such a comparison valid with respect to
its shape, but also with respect to its structure, which is absolutely unique among-
vertebrates, and very different to that of any other vertebrate gland, but
resembles in a striking- manner a glandular structure found in the uterus, both
of male and female scorpions.

The generative glands in Limulus, together with the liver-glands, form a
large glandular mass, situated in the head-region closely surrounding the central
nervous system, so that the. genital ducts pass from the head-region tailwards
to the operculum. In the scorpion they lie in the abdominal region, so that
their ducts pass headwards to the operculum.

Probably in the Palaeostraca the generative mass was situated in the cephalic
region as in Limulus, and it is probable that the remnant of it still exists in

THE EVIDENCE OF THE THYROID GLAND 21 7

Ammocoetes in the shape of the peculiar large cells packed together, with
pigment masses in between them, which form such a characteristic feature of
the glandular-looking material, which fills up the space between the cranial
walls and the central nervous system.

Finally, the relationship which has been known from time immemorial to
exist between the sexual organs and the thyroid in man and other animals, and
has hitherto been a mystery without any explanation, may possibly be the last
reminiscence of a time when the thyroid glands were the uterine glands of the
palaeostracan ancestor.

The consideration of the facial nerve, and the segments it supplies, still
further points to the origin of the Vertebrata from the Palfeostraca.

CHAPTER VI

THE EVIDENCE OF THE OLFACTORY APPARATUS

Fishes divided into Ainphirhina? and Monorhinfe. — Nasal tube of the lamprey.
— Its termination at the infundihulum. — The olfactory organs of the
scorpion group. — The camerostome. — Its formation as a tube. — Its deriva-
tion from a pair of antenna?. — Its termination at the true mouth. — Com-
parison with the olfactory tube of Ammocoetes. — Origin of the nasal tube
of Ammocoetes from the tube of the hypophysis. — Direct comparison of the
hypophysial tube with the olfactory tube of the scorpion group — Summary.

In the last chapter I finished the evidence given by the consideration
of the mesosomatic or opisthotic nerves, and the segments they
supplied. The evidence is strongly in accordance with that of
previous chapters, and not only confirms the conclusion that verte-
brates arose from some member of the Pakeostraca, but helps still
further to delimit the nature of that member. It is almost startling
to see how the hypothesis put forward in the second chapter, sug-
gested by the consideration of the nature of the vertebrate central
nervous system and of the geological record, has received stronger
and stronger confirmation from the consideration of the vertebrate
optic apparatus, the vertebrate skeleton, the respiratory apparatus,
and, finally, the thyroid gland. All fit naturally into a harmonious
whole, and give a feeling of confidence that a similar harmony will
be found upon consideration of the rest of the vertebrate organs.

Following naturally upon the segments supplied by the opisthotic
(mesosomatic) cranial nerves, we ought to consider now the segments
supplied by the pro-otic (prosomatic) cranial nerves, i.e. the segments
belonging to the trigeminal nerve-group in the vertebrate, and in the
invertebrate the segments of the prosoma with their characteristic
appendages. There are, however, in all vertebrates in this foremost
cranial region, in addition to the optic nerves, two other well-marked
nerves of special sense, the olfactory and the auditory. Of these,
the former are in the same class as the optic nerves, for they arise

THE EVIDENCE OF THE OLFACTORY APPARATUS 219

in the vertebrate from the supra-infundibular nerve-mass, and in the
invertebrate from the supra-CESophageal ganglia. The latter arise in
the vertebrate from the infra-infundibular nerve-mass, and, as the
name implies, are situated in the region where the pro-otic nerves
are contiguous to the opisthotic, i.e. at the junction of the prosomatic
and mesosomatic nerve-regions.

The chapter dealing with the evidence given by the olfactory
nerves and the olfactory apparatus ought logically to have followed
immediately upon the one dealing with the optic apparatus, seeing
that both these special sense-nerves belong to the supra-infundibular
segments in the vertebrate and to the supra- oesophageal in the
invertebrate.

I did not deal with them in that logical sequence because it was
necessary for their understanding to introduce first the conception of
modified appendages as important factors in any consideration of
vertebrate segments ; a conception which followed naturally after the
evidence afforded by the skeleton in Chapter III., and by the branchial
segments in Chapter IV. So, too, now, although the discussion of
the prosomatic segmentation ought logically to follow immediately
on that of the mesosomatic segmentation, I have determined to devote
this chapter to the evidence of the olfactory organs, because the
arguments as to the segments belonging to the trigeminal nerve-
group are so much easier to understand if the position of the olfactory
apparatus is first made clear.

In all vertebrates the nose is double and opens into the pharynx,
until we descend to the fishes, where the whole group Pisces has
been divided into two subsidiary groups, MonorhinaB and Ainphirhime,
according as they possess a median unpaired olfactory opening, or a
paired opening. The Monorhinoe include only the Cyclostomata — the
lampreys and hag-fishes.

In the lampreys the single olfactory tube ends blindly, while in
the hag-fishes it opens into the pharynx. In the lamprey, both in
Petromyzon and Ammoccetes, the opening of this nasal tube is a
conspicuous object on the dorsal surface of the head in front of the
transparent spot which indicates the position of the right median
eye. It is especially significant, as showing the primitive nature of
this median olfactory passage, that a perfectly similar opening in the

2 20 THE ORIGIN OF VERTEBRATES

same position is always found in the dorsal head-shields of all the
Cephalaspidse and Tremataspidse, as will be explained more fully in
Chapter X.

All the evidence points to the conclusion that the olfactory
apparatus of the vertebrate originated as a single median tube, con-
taining the special olfactory sense-epithelium, which, although median
and single, was innervated by the olfactory nerve of each side. The
external opening of this tube in the lamprey is dorsal. How does it
terminate ventrally ?

The ventral termination of this tube is most instructive and
suggestive. It terminates blindly at the very spot where the in-
fundibular tube terminates blindly and the notochord ends. After
transformation, when the Ammoccete becomes the Petromyzon, the
tube still ends blindly, and does not open into the pharynx as in
Myxine ; it, however, no longer terminates at the infundibulum, but
extends beyond it towards the pharynx.

This position of the nasal tube suggests that it may originally have
opened into the tube of the central nervous system by way of the
infundibular tube. This suggestion is greatly enhanced in value by
the fact that in the larval Amphioxus the tube of the central nervous
system is open to the exterior, its opening being known as the anterior
neuropore, and this anterior neuropore is situated at the base of a pit,
known as the olfactory pit because it is supposed to represent the
olfactory organ of other fishes.

Following the same lines of argument as in previous chapters,
this suggestion indicates that the special olfactory organs of the
invertebrate ancestor of the vertebrates consisted of a single median
olfactory tube or passage, which led directly into the oesophagus and
was innervated, though single and median, by a pair of olfactory
nerves which arose from the supra-cesophageal ganglia. Let us see
what is the nature of the olfactory organs among arthropods, and
whether such a suggestion possesses any probability.

The Olfactory Organs of the Scorpion Group.

At first sight the answer appears to be distinctly adverse, for it is
well known that in all the Insecta, Crustacea, and the large majority
of Arthropoda, the first pair of antenna3, often called the antennules,
are olfactory in function, and these are free-moving, bilaterally

THE EVIDENCE OF THE OLFACTORY APPARATUS 22 1

situated, independent appendages. Still, even here there is the strik-
ing fact that the nerves of these olfactory organs always arise from
the supra-cesophageal ganglia, although those to the second pair of
antenna? arise from the infra-cesophageal ganglia, just as the olfactory
nerves of the vertebrate arise from the supra-infundibular brain-mass.
Not only is there this similarity of position, but also a similarity of
structure in the olfactive lobes of the brain itself of so striking a cha-
racter as to cause Bellonci to sum up his investigations as follows : —

" The structure and connections of the olfactive lobes present the
same fundamental plan in the higher arthropods and in the verte-
brates. In the one, as in the other, the olfactory fibres form, with
the connecting fibres of the olfactory lobes, a fine meshwork, which,
consisting as it does of separate groups, may each one be called an
olfactory glomerulus."

He attributes this remarkable resemblance to a physiological
necessity that similarity of function necessitates similarity of structure,
for he considers it out of the question to suppose any near relationship
between arthropods and vertebrates.

Truly an interesting remark, with the one fallacy that relationship
is out of the question.

The evidence so far has consistently pointed to some member of the
pala?ostracan group as the ancestor of the vertebrates — a group which
had affinities both to the crustaceans and the arachnids ; indeed, many
of its members resembled scorpions much more than they resemble
crustaceans. The olfactory organs of the scorpions and their allies are,
therefore, more likely than any others to give a clue to the position of
the desired olfactory organs. In these animals and their allies paired
olfactory antenna? are not present, either in the living land-forms or
the extinct sea-scorpions, for all the antenna?-like, frequently chelate,
appendages seen in Pterygotus, etc. (Fig. 8), represent the chelicene, and
correspond, therefore, to the second pair of antenna? in the crustaceans.

What, then, represents the olfactory antenna? in the scorpions ? The
answer to this question has been given by Croneberg, and very strik-
ing it is. The two olfactory antenna? of the crustacean have combined
together to form a hollow tube at the base of which the mouth of the
animal is situated, so that the food passes along this olfactory passage
before it reaches the mouth. This organ is often called after Latreille,
the camerostome, sometimes the rostrum ; it is naturally median in
position and appears, therefore, to be an unpaired organ ; its paired

222

THE ORIGIN OF VERTEBRATES

cam

pr.ent

character is, of course, evident enough, for it is innervated by a pair of
nerves, and these nerves, as ought to be the case, arise from the supra-
oesophageal ganglia. In Galeodes it is a conspicuously paired antennae-
like organ (Fig. 94).

Croneberg has also shown that this rostrum, or camerostome, arises
embryologically as a pair of appendages similar to the other append-
ages. This last observation
of Croneberg has been con-
firmed by Brauer in 1894,
who describes the origin of
the upper lip, as he calls it,
in very similar terms, with-
out, however, referring to
Croneberg's paper. Crone-
berg further shows that this
foremost pair of antennas
not only forms the so-called
upper lip or camerostome,
but also a lower lip, for
from the basal part of the
camerostome there projects
on each side of the pharynx
a dependent accessory por-
tion, which in some cases
fuses in the middle line, and
forms, as it were, a lower lip.
The entosclerite belonging
to this dependent portion is
apparently the post - oral
entosclerite of Lankester and
Miss Beck.

At the base of the tubular
passage formed by this modified first pair of antennas the true mouth
is found opening directly into the dilated pharynx, the muscles
of which enable the act of suction to be carried out. The narrow
oesophagus leads out from the pharynx and is completely surrounded
by the supra- and infra-oesophageal nerve masses.

Huxley also describes the mouth of the scorpion in precisely the
same position (cf. o, Fig. 96).

Fig. 94. — Dobsal View of Brain and Came-
rostome op Galeodes.

cam., camerostome; pr. ent., pre-oral entoscle-
rite ; l.l., dependent portion of camerostome ;
ph., pharynx; al., alimentary canal; n. op.,
median optic nerves; pi., plastron; v.c,
ventral nerve chain ; 2, 3, second and third
appendages.

THE EVIDENCE OF THE OLFACTORY APPARATUS

223

In order to convey to my readers the antennae-like character of the
carnerostome in Galeodes (Fig. 101), and its position, I give a figure
(Fig. 94) of the organ from its dorsal aspect, after removal of the
cheliceras and their muscles. A side view of the same organ is given
in Fig. 95 to show the feathered termination of the carnerostome,
and the position of the dependent accessory portion {1.1.) (Crone-
berg's ' untere Anhang ') with its single long antenna-like feather.
In both figures the alimentary canal (al.) is seen issuing from the
conjoined supra- and infra-cesophageal mass.

As is seen in the figures, the bilateral character of the rostrum, as
Croneberg calls it, is apparent not only in its feathered extremity
but also in its chitinous covering, the softer median dorsal part (left

p"er\t

Fig. 95. — Lateral View of Brain and Camerostome of Galeodes.

gl. supr. ces., supra-oesophageal ganglion; gl. infr. ces., infra-cesophageal ganglion.
The rest of the lettering same as in Fig. 94.

white in figure) being bounded by two lateral plates of hard chitin,
which meet in the middle line near the extremity of the organ. In
all the members of the scorpion group, as is clearly shown in Crone-
berg's figures, the rostrum or carnerostome is built up on the same
plan as in Galeodes, though the antenna-like character may not be
so evident.

When we consider that the first pair of antennae in the crustaceans
are olfactory in function, Croneberg's observations amount to this —

In the arachnids and their allies the first pair of antennae form
a pre-oral passage or tube, olfactory in function ; the small mouth,
which opens directly into the pharynx, being situated at the end
of this olfactory passage.

224

THE ORIGIN OF VERTEBRATES

Croneberg's observatiuns and conclusions are distinctly of very
groat importance in bringing the arachnids into line with the crus-
taceans, and it is therefore most surprising that they are absolutely
ignored by Lankester and Miss Beck in their paper published in
1883, in which Latreille only is mentioned with respect to this
organ, and his term " camerostome," or upper lip, is used throughout,
in accordance with the terminology in Lankester's previous paper.
That this organ is not only a movable lip or tongue, but essentially
a sense-organ, almost certainly of smell and taste, as follows from
Croneberg's conclusions, is shown by the series of sections which I
have made through a number of young Thelyphonus (Fig. 102).

pr em

. / Hyp

Olf pass

Fig. 96. — Median Sagittal Section through a Young Thelyphonus.

I give in Fig. 96 a sagittal median section through the head-end
of the animal, which shows clearly the nature of Croneberg's con-
ception. At the front end of the body is seen the median eye (cc),
u is the mouth, Ph. the pharynx, ces. the narrow cesophagus, com-
pressed between the supra-oesophageal (swpr. ces.) and infra-cesopha-
geal (infr. ces.) brain mass, which opens into the large alimentary
canal (A I.) ; Olf. pass, is the olfactory passage to the mouth, lined
with thick- set, very fine hairs, which spring from the hypostome
(Hyp.) as well as from the large conspicuous camerostome (Cam.),
which limits this tube anteriorly. The space between the came-
rostome and the median eye is filled up by the massive chelicerse,
which are not shown in this section, as they begin to appear in the

THE EVIDENCE OF THE OLFACTORY APPARATUS 225

sections on each side of the median one. The muscles of the pharynx
and the muscles of the camerostome are attached to the pre-oral
entosclerite (pr. ent.). The post-oral entosclerite is shown in section
as post. ent. The dorsal blood-vessel, or heart, is indicated at H.

In Tig. 97 I give a transverse section through another specimen
of the same litter, to show the nature of this olfactory tube when cut
across. Both sections show most clearly that we are dealing here with
an elaborate sense-organ, the surface of which is partly covered with
very fine long hairs, partly, as is seen in the figure, is composed of
long, separate, closely-set sense-rods (bat.), wTell protected by the
long hairs which project on every side in front of them, which recall
to mind Bellonci's figure of the ' batonnets olfactives ' on the antennae
of Sphasroma. Finally, we have the observation of Blanchard quoted
by Huxley, to the effect that this camerostome is innervated by
nerves from the supra-cesophageal ganglia which are clearly bilateral,
seeing that they arise from the ganglion on each side and then unite
to pass into the camerostome ; in other words, paired olfactory nerves
from the supra-cesophageal ganglia.

These facts demonstrate with wonderful clearness that in one
group of the Arthropoda the olfactory autennae have been so modi-
fied as to form an olfactory tube or passage, which leads directly
into the mouth and so to the oesophagus of the animal, and, strikingly
enough, this group, the Arachnida, is the very one to which the
scorpions belong.

If for any cause the mouth 0 in Fig. 96 were to be closed, then
the olfactory tube (olf. i^ciss.) might still remain, owing to its impor-
tance as the organ of smell, and the olfactory tube would terminate
blindly at the very spot where the corresponding tube does terminate
in the vertebrate, according to the theory put forward in this book.

The Olfactory Tube of Ammoccetes.

In all cases where there is similarity of topographical position
in the organs of the vertebrate and arthropod we may expect also to
find similarity of structure. At first sight it would appear as though
such similarity fails us here, for a cross-section of the olfactory tube
in Petromyzon represents an elaborate organ such as is shown in Fig.
98, very different in appearance to the section across the olfactory
passage of a young Thelyphonus given in Fig. 97.

THE ORIGIN OF VERTEBRATES

few

»1&

Fig. 97. — Teansveese Section theough the Olfactoey Passage of a Young

Thelyphonus.

1 and 2, sections of first and second appendages.

--cart.

Fig. 98.— Teansveese Section theough the Olfactoey Passage of Peteomyzon.

cart., nasal cartilage.

THE EVIDENCE OF THE OLFACTORY APPARATUS

227

As is seen, it is difficult to see any connection between these
folds of olfactory epithelium and the simple tube of the scorpion.
But in the nose, as in all other parts of the head-region of the
lamprey, remarkable changes take place at transformation, and
examination of the same tube in Ammococtes demonstrates that the
elaborate structure of the adult olfactory organ is actually derived
from a much simpler form of organ, represented in Fig. 99. Here,
in Ammoccetes, the section is no longer strikingly different from that
of the Thelyphonus organ, but, instead, most strikingly similar to it.
Thus, again, it is shown that this larval form of the lamprey gives

■ cart

Fig. 99. — Transverse Section through the Olfactory Passage op Ammoccetes.

cart., nasal cartilage.

more valuable information as to vertebrate ancestry than all the
rest of the vertebrates put together.

Still, even now the similarity between the two organs is not
complete, for the tube in the lamprey opens on to the exterior on the
dorsal surface of the head, while in the scorpion tribe it is situated
ventrally, being the passage to the mouth and alimentary canal. In
accordance with this there is no sign of any opening on the dorsal
carapace of any of the extinct sea-scorpions or of the living land-
scorpions, such as is so universally found in the cephalaspids, trema-
taspids, and lampreys. Here is a discrepancy of an apparently
serious character, yet so wonderfully does the development of the
individual recapitulate the development of the race, that this very
discrepancy becomes converted into a triumphant vindication of the

2 28 THE ORIGIN OF VERTEBRATES

correctness of the theory advocated in this book, as soon as we turn
our attention to the development of this nasal tube in the lamprey.

We must always remember not only the great importance of a lar-
val stage for the unriddling of problems of ancestry, but also the great
advantage of being able to follow more favourably any clues as to
past history afforded by the development of the larva itself, owing
to the greater slowness in the development of the larva than of the
embryo. Such a clue is especially well marked in the course of
development of Ammoccetes according to Kupffer's researches, for
he finds that when the young Ammocoetes is from 5 to 7 mm. in
length, some time- after it has left the egg, when it is living a free
larval life, a remarkable series of changes takes place with consider-
able rapidity, so that we may regard the transformation which takes
place at this stage, as in some degree comparable with the great trans-
formation which occurs when the Ammoccetes becomes a Petromyzon.

All the evidence emphasizes the fact that the latter transformation
indicates the passage from a lower into a higher form of vertebrate,
and is to be interpreted phylogenetically as an indication of the
passage from the Cephalaspidian towards the Dipnoan style of fish.
If, then, the former transformation is of the same character, it would
indicate the passage from the Paheostracan to the Cephalaspid.

What is the nature of this transformation process as described
by Kupffer ?

It is characterized by two most important events. In the first
place, up to this time the oral chamber has been cut off from the
respiratory chamber by a septum — the velum — so that no food could
pass from the mouth to the alimentary canal. At this stage the
septum is broken through, the oral chamber communicates with the
respiratory chamber, and the velar folds of the more adult Ammocoetes
are left as the remains of the original septum. The other striking
change is the growth of the upper lip, by which the orifice of the nasal
tube is transferred from a ventral to a dorsal position. Fig. 100,
taken from Kupffer's paper, represents a sagittal section through an
Ammoccetes 4 mm. long; l.l. is the lower lip, u.l. the upper lip, and,
as is seen, the short oral chamber is closed by the septum, rel. Open-
ing ventrally is a tube called the tube of the hypophysis, Hy., which
extends close up to the termination of the infundibulum. On the
anterior surface of this tube is the projection called by Kupffer the
olfactory plakode. At this stage the upper lip grows with great

THE EVIDENCE OF THE OLFACTORY APPARATUS 229

rapidity and thickens considerably, thus forcing the opening of the
hypophysial tube more and more dorsalwards, until at last, in the full-
grown Ammoccetes, it becomes the dorsal opening of the nasal tube,
as already described. Here, then, in the hypophysial tube we have
the original position of the olfactory tube of the vertebrate ancestor,
and it is significant, as showing the importance of this organ, to find
that such a hypophysial tube is characteristic of the embryological
development of every vertebrate, whatever may be the ultimate form
of the external nasal orifices.

The single median position of the olfactory organ in the Cyclo-
stomata, in contradistinction to its paired character in the rest of the

v xv\\ IX x

Hu ui Or 11 vel

Fig. 100. — Ganglia of the Cranial Nerves of an Ammoccetes, i mm. in length,

PROJECTED ON TO THE MEDIAN PLANE. (After KUPFFER.)

A-B, the line of epibranchial ganglia; an., auditory capsule; nc, notochord ; Hy.,
tube of hypophysis ; Or., oral cavity; u.l., upper lip ; l.l. lower lip; vel., septum
between oral and respiratory cavities ; V., VII., IX., X., cranial nerves ; x.,
nerve with four epibranchial ganglia.

vertebrates, has always been a stumbling-block in the way of those
who desired to consider the Cyclostomata as degenerated Selachians,
for the origin of the olfactory protuberance, as a single median
plakode, seemed to indicate that the nose arose as a single organ and
not as a paired organ.

On the other hand, the two olfactory nerves of Ammoccetes
compare absolutely with the olfactory nerves of other vertebrates,
and force one to the conclusion that this median organ of Ammo-
ccetes arose from a pair of bilateral organs, which have fused in the
middle line.

The comparison of this olfactory organ with the camerostome

230

THE ORIGIN OF VERTEBRATES

Fig. 101.— Galcodes. (Prom the Royal Natural History.)

THE EVIDENCE OF THE OLFACTORY APPARATUS

23l

gives a satisfactory reason for its appearance in the lowest verte-
brates as an unpaired median organ ; equally so, the history of the
camerostoine itself supplies the reason why the olfactory nerves are
double, why the organ is in reality a paired organ and not a single

Fig. 102. — Thelyphonus. (From the Royal Natural History.)

median one. Thus, in a sense, the grouping of the fishes into Mono-
rhinae and Amphirhinse has not much meaning, seeing that the
olfactory organ is in all cases double.

The evidence of the olfactory organs in the vertebrate not only
confirms, in a most striking manner, the theory of the origin of the

232 THE ORIGIN OF VERTEBRATES

vertebrate from the Palaeostracan, but points indubitably to an origin
from a scorpion-like rather than a crustacean-like stock. To com-
plete the evidence, it ought to be shown that the ancient sea-scorpions
did possess an olfactory passage similar to the modern land-scorpions.
The evidence on this question will come best in the next chapter,
where I propose to deal with the prosomatic appendages of the Palse-
ostracan group.

Summary.

The vertebrate olfactory apparatus commences as a single median tube
which terminates dorsally in the lamprey, and is supplied by the two olfactory
nerves which arise from the supra-infundibular portion of the brain. It is
a long-, tapering- tube which passes ventrally and terminates blindly at the
infundibulum in Ammocoetes. The dorsal position of the nasal opening is not
the original one, but is brought about by the growth of the upper lip. The
nasal tube originally opened ventrally, and was at that period of development
known as the tube of the hypophysis.

The evidence of Ammocoetes thus goes to show that the olfactory
apparatus started as an olfactory tube on the ventral side of the animal, which
led directly up to, and probably into, the oesophagus of the original alimentary
canal of the palaeostracan ancestor.

Strikingly enough, although in the crustaceans the first pair of antenna?
form the olfactory organs, no such free antennas are found in the arachnids,
but they have amalgamated to form a tube or olfactory passage, which leads
directly into the mouth and oesophagus of the animal.

This olfactory passage is very conspicuous in all members of the scorpion
group, and, like the olfactory tube of the vertebrate, is innervated by a pair of
nerves, which resemble those supplying the first pair of antenna? in crustaceans
as to their origin from the supra-cesophageal ganglia.

This nasal passage, or tube of the hypophysis, corresponds in structure and
iu position most closely with the olfactory tube of the scorpion group, the only
difference being- that in the latter case it opens directly into the oesophagus,
while in the former, owing to the closure of the old moutli, it cannot open into
the infundibulum.

The evidence of the olfactory apparatus, combined with that of the optic
apparatus, is most interesting, for, whereas the former points indubitably to an
ancestor having scorpion-like affinities, the structure of the lateral eyes points
distinctly to crustacean, as well as arachnid, affinities.

Taking the two together the evidence is extraordinarily strong that the
vertebrate arose from a member of the palasostracan group with marked
scorpion-like affinities.

CHAPTER VII

THE PROS OM A TIC SEGMENTS OF LIMULUS AND ITS ALLIES

Comparison of the trigeminal with the prosomatic region. — The prosomatic
appendages of the Gigantostraca. — Their number and nature. — Endognaths
and ectognath. — The metastoma.— The coxal glands. — Prosomatic region
of Eurypterus compared with that of Ammocoetes. — Prosomatic segmenta-
tion shown by muscular markings on carapace. — Evidence of coelomic
cavities in Limulus. — Summary.

The derivation of the olfactory organs of the vertebrate from the
olfactory antennae of the arthropod in the last chapter is confirmatory
proof of the soundness of the proposition put forward in Chapter IV.,
that the segmentation in the cranial region of the vertebrate was
derived from that of the prosomatic and mesosomatic regions of the
palseostracan ancestor. Such a segmentation implies a definite series
of body-segments, corresponding to the mesomeric segmentation of
the vertebrate, and a definite series of appendages corresponding to
the splanchnic segmentation of the vertebrate.

Of the foremost segments belonging to the supra-oesophageal
region characterized by the presence of the median eyes, of the lateral
eyes, and of the olfactory organs, a wonderfully exact replica has
been shown to exist in the pineal eyes, the lateral eyes, and the
olfactory organ of the vertebrate, belonging, as they all do, to the
supra-infundibular region.

Of the infra- cesophageal segments belonging to the prosoraa and
mesosoma respectively, the correspondence between the mesosomatic
segments carrying the branchial appendages and the uterus, with
those in the vertebrate carrying the branchiae and the thyroid gland
respectively, has been fully proved in previous chapters.

There remain, then, only the segments of the prosomatic region
to be considered, a region which, both in the vertebrate and inver-
tebrate, is never respiratory in function but always masticatory, such

234 TM£ ORIGIN OF VERTEBRATES

mastication being performed in Limulns and its allies by the muscles
which move the foot-jaws or gnathites, which are portions of the
prosomatic appendages specially modified for that purpose, and in the
vertebrates by the masticatory muscles, which are always innervated
by the trigeminal or Vth cranial nerve. This comparison implies
that the motor part of the trigeminal nerve originally supplied the
prosomatic appendages.

The investigations of van Wijhe and of all observers since the
publication of his paper prove that in this trigeminal region, as in the
vagus region, a double segmentation exists, of which the ventral or
splanchnic segments, corresponding to the appendages in the inver-
tebrate, are supplied by the trigeminal nerves, while the dorsal or
somatic segments, corresponding to the somatic segments in the
invertebrate, are supplied by the Illrd or oculomotor and the IVth
or trochlear nerves — nerves which supply muscles moving the lateral
eyes.

In accordance, then, with the evidence afforded by the nerves of
the branchial segments, it follows that the muscles supplied by the
motor part of the trigeminal ought originally to have moved the ap-
pendages belonging to a series of prosomatic segments. On the other
hand, the eye-muscles ought to have belonged to the body-part of the
prosomatic segments, and must therefore have been grouped origi-
nally in a segmental series corresponding to the prosomatic appendages.

The evidence for and against this conclusion will be the subject
of consideration in this and the succeeding chapters. At the outset
it is evident that any such comparison necessitates an accurate know-
ledge of the number of the prosomatic segments in the Gigantostraca
and of the nature of the corresponding appendages.

In all this group of animals, the evidence as to the number of
segments in either the prosomatic or mesosomatic regions is given
by-

1. The number of appendages.

2. The segmental arrangement of the muscles of the prosoma or
mesosoma respectively.

3. The segmental arrangement of the ccelomic or head-cavities.

4. The divisions of the central nervous system, or neuromeres,
together with their outgoing segmental nerves.

It follows, therefore, that if from any cause the appendages are
not apparent, as is the case in many fossil remains, or have dwindled

PROSOMATIC SEGMENTS OF LIMULUS 235

away and become insignificant, we still have the muscular, ccelomic,
and nervous arrangements left to us as evidence of segmentation in
these animals, just as in vertebrates.

In this prosomatic region, we find in Limulus the same tripartite
division of the nerves as in the mesosomatic region, so that the
nerves to each segment may be classed as (1) appendage-nerve ;
(2) sensory or dorsal somatic nerve, supplying the prosomatic cara-
pace ; (3) motor or ventral somatic nerve, supplying the muscles of
the prosoma, and containing possibly some sensory fibres. The main
difference between these two regions in Limulus consists in the closer
aggregation of the prosomatic nerves, corresponding to the concentra-
tion of the separate ganglia of origin in the prosomatic region of the
brain.

The number of prosomatic segments in Limulus is not evident
by examination of the prosomatic carapace, so that the most reliable
guide to the segmentation of this region is given by the appendages,
of which one pair corresponds to each prosomatic segment.

The number of such segments, according to present opinion, is
seven, viz. : —

(1) The foremost segment, which bears the chelicerae.
(2, 3, 4, 5, 6) The next five segments, which carry the paired
locomotor appendages ; and

(7) The last segment, to which belongs a small abortive pair of
appendages, known by the name of the chilaria, situated between the
last pair of locomotor appendages and the operculum or first pair of
mesosomatic appendages. These appendages are numbered from 1-7
in the accompanying drawing (Fig. 103).

Of these seven pairs of appendages, the significance of the first
and the last has been matter of dispute. With respect to the first
pair, or the chelicerae, the question has arisen whether their nerves
belong to the infra-oesophageal group, or are in reality supra-
cesophageal.

It is instructive to observe the nature and the anterior position of
this pair of appendages in the allied sea-scorpions, especially in Ptery-
gotus, where the only chelate organs are found in these long, antennae-
like chelicerae. In Slimonia and in Stylonurus they are supposed by
Woodward to be represented by the small non-chelate antennas seen
in Fig. 8, B and C (p. 27), taken from Woodward. If such is the case,
then these figures show that a pair of appendages is missing in each

236 THE ORIGIN OF VERTEBRATES

of these forms, for they possess only five free prosomatic appendages
instead of six, as in Limulus and in Pterygotus. Similarly, Wood-
ward only allowed five appendages for Pterygotus, so that his restora-
tions were throughout consistent. Schmidt, in Pterygotus osiliensis
has shown that the true number was six, not five, as seen in his
restoration given in Fig. 8, A (p. 27).

With respect to Eurypterus, Schmidt figures an exceedingly

Fig. 103. — Ventral Surface op Limulus. (Taken from Kishinouye.)

The gnathic bases of the appendages have been separated from those of the other
side to show the promesosternite or endostoma (End.).

minute pair of antennae between the coxal joints of the first pair of
appendages, thus making six pairs of appendages. Gerhard Holm,
however, in his recent beautiful preparations from Schmidt's specimens
and others collected at Bootzikiill, has proved most conclusively that
the chelicera3 of Eurypterus were of the same kind as those of
Limulus. I reproduce his figure (Fig. 104) showing the small chelate
chelicerce (1) overhanging the mouth orifice, just as in Limulus or in
Scorpio.

PROS DMA TIC SEGMENTS OF LIMULUS

?37

So, also, since Woodward's monograph, Laurie has discovered in
Slimonia acuminata a small median pair of chelate appendages
exactly corresponding to the chelicerae of Limulus, or of Eurypterus,
or of Scorpio. We may, therefore, take it for granted that such was
also the case in Stylonurus, and that the foremost pair of proso-
matic appendages in all these extinct sea-scorpions were in the same
position and of the same character as the cheliceral of the scorpions.

In the living scorpion and in Limulus the nerves to this pair of

Fig. 104. — Eurypterus Fischeri. (From Holm.)

appendages undoubtedly arise from the foremost prosomatic ganglia,
and the reason why they appear to beloug to the supra- oesophageal
brain-mass has been made clear by Brauer's investigations on the
embryology of Scorpio ; for he has shown that the cheliceral ganglia
shift from the ventral to the dorsal side of the oesophagus during
development, thus becoming pseudo-supra-cesophageal, though in
reality belonging to the iufra-cesophageal ganglia. This cheliceral
pair of appendages is, in all probability, homologous with the second
pair of antennas in the Crustacea.

2^8 THE ORIGIN OF VERTEBRATES

I conclude, then, that the chelicera? must truly be included in
the pro-somatic group, but that they stand in a somewhat different
category to the rest of the prosomatic appendages, inasmuch as they
take up a very median anterior and somewhat dorsal position, and
their ganglia of origin are also exceptional in position.

Next for consideration come the chilaria (7 in Fig. 103), which
Lankester did not consider to belong to appendages at all, but to
be a peculiar pair of sternites. Yet their very appearance, with
their spinous hairs corresponding to those of the other gnathites and
their separate nerve-supply, all point distinctly to their being a
modified pair of appendages, and, indeed, the matter has been placed
beyond doubt by the observations of Kishinouye, who has found
embryologically that they arise in the same way as the rest of the
prosomatic appendages, and belong to a distinct prosomatic segment,
viz. the seventh segment. In accordance with this, Brauer has found
that in the scorpion there is in the embryo a segment, whose ap-
pendages degenerate, which is situated between the segment bearing
the last pair of thoracic appendages and the genital operculum — a
segment, therefore, comparable in position to the chilarial segment of
Limulus.

Coming now to the five locomotor appendages, we find that they
resemble each other to a considerable extent in most cases, with,
however, certain striking differences. Thus in Limulus they are
chelate, with their basal joints formed as gnathites, except in the
case of the fifth appendage, in which the extremity is modified for
the purpose of digging in the sand. In Pterygotus, Slimonia, Euryp-
terus, the first four of these appendages are very similar, and are
called by Huxley and Woodward endognaths; in all cases they
possess a basal part or sterno-coxal process, which acts as a gnathite
or foot-jaw, and a non-chelate tactile part, which possesses no pre-
hensile power, and in most cases could have had no appreciable
share in locomotion, called by Huxley and Woodward the palpus.
These small palps were probably retractile, and capable of being
withdrawn entirely under the hood. The fifth appendage is usually
different, being a large swimming organ in Pterygotus, Eurypterus,
and Slimonia (Figs. 8 and 104), and is known as the ectognath.

Finally, in Drcpanopterus Bembycoides, as stated by Laurie, all
five locomotor appendages are built up after the same fashion, the
last one not being formed as a paddle-shaped organ or elongated as

PROSOMATIC SEGMENTS OF LIMULUS 239

in Stylonurus, but all five possess no special locomotor or prehensile
power. According to Laurie this is a specially primitive form of the
group.

It is significant to notice from this sketch that with the absence
of special prehensile terminations such as chelas, or the absence of
special locomotor functions such as walking or swimming, these
appendages tend to dwindle and become insignificant, taking up the
position of mere feelers round the mouth, and at the same time are
concentrated and pressed closely together, so that their appendage-
nerves must also be close together.

This sketch therefore shows us that —

Of the six foremost prosomatic appendages, the chelicerre and the
four endognaths were, at the time when the vertebrates first appeared,
in very many cases dwindling away ; the latter especially no longer
functioned as locomotor appendages, but were becoming more and
more mere palps or tentacles situated round the mouth, which could
by no possibility afford any help to locomotion.

On the contrary, the sixth pair of appendages — the ectognaths —
remained powerful, being modified in many cases into large oar-like
limbs by which the animal propelled itself through the water.

It is a striking coincidence that those ancient fishes, Ptericthys
and Bothriolepis, should have possessed a pair of large oar-like
appendages.

At this time, then, in strong contrast to the endognaths, the
ectognaths, or sixth pair of appendages, remained strong and vigo-
rous. What about the seventh pair, the chilaria of Limulus ?

Of all the prosomatic appendages these are the most interesting
from the point of view of my theory, for whereas in the scorpion of
the present day they have dwindled away and left no trace except in
the embryo, in the sea-scorpions of old, far from dwindling, they
had developed and become a much more important organ than the
chilaria of Limulus.

In all these animals a peculiarly striking and unique structure is
found in this region known by the name of the metastoma, or lip-plate
(Figs. 8 and 104 (7)) ; it is universally considered to be formed by
the fusion of the two chilarial appendages.

All observers are agreed that this lip-plate was freely movable.
Nieskowski considers that the movement of the metastoma was
entirely in a vertical direction, whereby the cleft which is seen

240

THE ORIGIN OF VERTEBRATES

between the basal joints of all the pairs of locomotor appendages
could be closed from behind. Woodward says it no doubt represents
the labium, and served more effectually to enclose the posterior part
of the buccal orifice, being found exteriorly to the toothed edges of the
ectognaths or maxillipedes. Schmidt agrees with Nieskowski, and

M.obl.

Fig. 105. — Diagram of Sagittal Median Section through A, Limulus, B,

Eurypterus.

looks on the mestasoma as forming a lower lip within which the
bases of the ectognaths worked.

Quite recently Gerhard Holm has worked over again the very
numerous specimens of Eurypterus Fischeri, which are obtainable at
Rootzikull, and has thrown new light on the relation of the metas-
toma to the mouth-parts. His preparations show clearly that the
true lower lip of Eurypterus was not the metastoma, for when the
metastoma is removed another plate {End., Pig. 105, B) situated

PROSOMATIC SEGMENTS OF LIMULUS 24 1

internally to it is disclosed, which, in his view, corresponds to the
sternite between the bases of the pro-somatic appendages in Lirnu-
lus, i.e.. to the sternite called by Lankester, the pro-mesosternite (End.,
Fig. 103). This inner plate formed with the metastoma ((7) Fig.
105) and the ectognaths (6) a chamber closed posteriorly, within
which the bases of the ectognaths worked. In other words, the
removal of the metastoma discloses in Eurypterus the true anterior
ventral surface of the animal which corresponds to that of Limulus,
or of the scorpion group, with its pro-mesosternite and laterally
attached gnathites or sterno-coxal processes. To this inner plate or
pro-mesosternite Holm gives the name of endostoma.

To the anterior edge of the endostoma a thinner membrane is
attached which passes inwards in the direction of the throat, and
forms, therefore, the lower lip (Hyp., Fig. 105, B) of the passage of
the mouth (olf.p.). This membrane bears upon its surface a tuft of
hairs, which he thought were probably olfactory in function. Con-
sequently, in his preliminary communication, he describes this lower
lip as forming, in all probability, an olfactory organ ; in his full
communication he repudiates this suggestion, because he thinks it
unlikely that such an organ would be situated within the mouth. I
feel sure that if Holm had referred to Croneberg's paper, and seen
how the true mouth in all the scorpion group is situated at the base
of an olfactory passage, he would have recognized that his first sug-
gestion is in striking accordance with the nature of the entrance to
the mouth in other scorpions.

That Eurypterus also possessed a camerostome (ram.) seems to
follow of necessity from its evident affinities both with Limulus
and the scorpions. We see, in fact, that the mouth of these old sea-
scorpions was formed after the fashion of Limulus, surrounded by
masticatory organs in the shape of foot-jaws, and yet foreshadowed
that of the scorpion, so that an ideal sagittal section of one of these
old pakeostracan forms would be obtained by the combination of
actual sagittal sections through Limulus and a member of the scorpion
group, with, at the same time, a due recognition of Holm's researches.
Such a section is represented in Fig. 105, B, in which I have drawn
the central nervous system and its nerves, the median eyes (C.E.),
the olfactory organs (Cam.), the pharynx (Ph.), oesophagus (ces.), and
alimentary canal (Al.), but have not tried to indicate the lateral eyes.
I have represented the prosomatic appendages by numbers (1-7), and

242 THE ORIGIN OF VERTEBRATES

the foremost mesosomatic segments by numbers (8-13). I have
placed the four endognaths and the nerves going to them close
together, and made them small, mere tentacles, in recognition of the
character of these appendages in Eurypterus, and have indicated the
position and size of the large ectognath, with its separate nerve,
by (6). If among the ancient Eurypterus-like forms, which were
living at the time when vertebrates first appeared, there were some
in which the ectognaths also had dwindled to a pair of tentacles,
then such animals would possess a prosomatic chamber formed by
a metastoma or accessory lip, within which were situated five pairs
of short tactile appendages or tentacles. If the vertebrate were
derived from such an animal, then the trigeminal nerve, as the
representative of these prosomatic appendage-nerves, ought to be
found to supply the muscles of this accessory lip and of these five
pairs of tentacles in the lowest vertebrate.

This prosomatic or oral chamber, as it might be called, was limited
posteriorly by the fused metastoma (7) and operculum (8), so that if
in the same imaginary animal one imagines that the gill-chambers,
instead of being separate, are united to form one large respiratory
chamber, then, in such an animal, a prosomatic oral chamber, in
which the prosomatic appendages worked, would be separated from
a mesosomatic respiratory chamber by a septum composed of the
conjoined basal portions of the mesosomatic operculum and the
prosomatic metastoma, as indicated in the diagram. In this septum
the nerves to the last prosomatic appendage (equivalent to the last
part of the trigeminal in the vertebrate) and to the first mesosomatic
(equivalent to the thyroid part of the facial) would run, as shown in
the figure, close together in the first part of their course, and would
separate when the ventral surface was reached, to pass headwards
and tailwards respectively.

The Coxal Glands.

One more characteristic of these appendages requires mention,
and that is the excretory glands situated at the base of the four
endognaths known as the coxal glands. These glands are the main
excretory organs in Limulus and the scorpions, and extend into the
basal segments or coxae of the four endognaths, not into those of the
ectognaths or the chilaria (or metastoma). Hence their name, coxal

PROSOMATIC SEGMENTS OF LIMULUS 243

glands ; and, seeing the importance of the excretory function, it is
likely enough that they would remain, even when the appendages
themselves had dwindled away. With the concentration and
dwindling of the endognaths these coxal glands would also be con-
centrated, so that in the diagram (Fig. 105) they would rightly be
grouped together in the position indicated (cox. gl.).

Such a diagram indicates the position of all the important organs
of the head-region except the special organs for taste and hearing.
These, for the sake of convenience, I propose to take separately, in
order at this stage of my argument not to overburden the simplicity
of the comparison I desire to make with too much unavoidable detail.

The Peosomatic Eegion of Ammoccetes.

Let us now compare this diagram with that of the corresponding-
region in Ammoccetes and see whether or no any points of similarity
exist.

With respect to this region, as in so many other instances already
mentioned, Ammoccetes occupies an almost unique position among
vertebrates, for the region supplied by the trigeminal nerve — the
prosoinatic region — consists of a large oral chamber which was
separated from the respiratory chamber in the very young stage by
a septum which is subsequently broken through, and so the two
chambers communicate.

This chamber is bounded by the lower lip ventrally, the upper
lip and trabecular region dorsally, and the remains of the septum or
velum laterally and posteriorly. It contains a number of tentacles
arranged in pairs within the chamber so as to form a sieve-like fringe
iuside the circular mouth ; of these, the ventral pair are large, fused
together, and attached to the lower lip.

All the muscles belonging to this oral chamber are of the
visceral type, and are innervated by the trigeminal nerve. In
accordance with the evidence obtained up to this point this means
that such an oral chamber was formed by the prosomatic appendages
of the invertebrate ancestor, similarly to the oral chamber just figured
for Eurypterus.

This chamber in the fall-grown Ammoccetes is not only open to
the respiratory chamber, but is bounded by the large upper lip (U.L.,
Fig. 106, D). On the dorsal surface of this region, in front of the

244 THE ORIGIN OF VERTEBRATES

pineal eye (C.E.), is the most conspicuous opening of the olfactory
tube (Na.), which olfactory tube passes from the dorsal region to the
ventral side to terminate blindly at the very spot where the infun-
dibulum comes to the surface of the brain. Here, also, is situated
that extraordinary glandular organ known as the pituitary body
(Pit.). A sagittal section, then, in diagram form, of the position
of parts in the full-grown Ammocu'tes, would bo represented as in
Fig. 106, D.

But, as argued out in the last chapter, the diagram of the adult
Ammocoetes must be compared with that of a cephalaspidian fish ;
the diagram of the palaiostracan must be compared with the larval
condition of Ammocoetes. In other words, Fig. 106, B, must be
compared with Fig. 106, C, which represents a section through the
larval Ammocoetes as it would appear if it reached the adult con-
dition without any forward growth of the upper lip or any breaking
through of the septum between the oral and respiratory chambers.
The striking similarity between this diagram and that of Euryp-
terus becomes immediately manifest even to the smallest details.
The only difference between the two, except, of course, the notochord,
consists in the closure of the mouth opening (o), in Fig. 106, B, by
which the olfactory passage (off. ^?.) of the scorpion becomes con-
verted into the hypophysial tube (Sy.), Fig. 106, C, and later into
the nasal tube {Na.), Fig. 106, D, of the full-grown Ammocoetes.
That single closure of the old mouth is absolutely all that is
required to convert the Euryptems diagram into the Ammocoetes
diagram.

Such a comparison immediately explains in the simplest manner
a number of anatomical peculiarities which have hitherto been among
the great mysteries of the vertebrate organization. For not only
do the median eyes (C.E.) correspond in position in the two diagrams,
and the infundibular tube (Inf.) and the ventricles of the brain
(CO.) correspond to the oesophagus (ces.) and the cephalic stomach
(Al), as already fully discussed ; but even in the very place wdiere the
narrow oesophagus opened into the wider chamber of the pharynx
(Ph.), there, in all the lower vertebrates, the narrow infundibular tube
opens into the wider chamber of the membranous saccus vasculosus (sac.
vase). This is the last portion of the membranous part of the tube of
the central nervous system which has not received explanation in the
previous chapters, and now it is seen how simple its explanation is,

PROSOMATIC SEGMENTS OF LIMULUS

245

M.obl
0 <*■?• _L

n.lV

Pit. / 2 : 3 ** G
sac. vase.

n.JV*

^^^■^^r^7Tv^^-777"-^^ NC

Fig. 100.— Diagram of Sagittal Median Section through B, Eurypterus ;
C, Larval Ammoccetes ; D, Full-grown Ammoccetes.

246 THE ORIGIN OF VERTEBRATES

how natural its presence — it represents the old pharyngeal chamber
of the pahcostracan ancestor.

Next among the mysteries requiring explanation is the pituitary
body, that strange glandular organ always found so closely attached
to the brain in the infundibular region that when it is detached in
taking out the brain it leaves the infundibular canal patent right into
the Illrd ventricle. A comparison of the two diagrams indicates
that such a glandular organ (Pit.), Fig. 106, C, was there because the
coxal excretory glands (cox. gl.), Fig. 106, B, were in a similar
position in the palseostracan ancestor — that, indeed, the pituitary
body is the descendant of the coxal glands.

Finally, the diagrams not only indicate how the mesosomatic
appendage-nerves supplying in the one case the operculum and the
respiratory appendages correspond to the respiratory group of nerves,
VII., IX., X., supplying in the other case the thyroid, hyoid, and
branchial segments, but also that a similar correspondence exists
between the prosomatic appendage-nerves in the one case and the
trigeminal nerve in the other ; a correspondence which supplies the
reason why in the vertebrate a septum originally existed between an
oral and respiratory chamber.

Such a comparison, then, leads directly to the suggestion that the
trigeminal nerve originally supplied the prosomatic appendages, such
appendages being: 1. The metastoma, which has become in Ammo-
ccetes the lower lip supplied by the velar or mandibular branch of the
trigeminal nerve (7) ; 2. The ectognath, which has become the large
median ventral tentacle, called by Eathke the tongue, supplied by the
tongue nerve (6) ; 3. The endognaths, which have been reduced to
tentacles and are supplied by the tentacular branch of the trigeminal
nerve (2, 3, 4, 5).

I have purposely put these two diagrams of the larval Ammo-
ccetes and of Eurypterus before the minds of my readers at this early
stage of my argument, so as to make what follows more understand-
able. I propose now to consider fully each one of these suggestive
comparisons, and to see whether or no they are in accordance with
the results of modern research.

In the first instance, the diagrams suggest that the trigeminal
nerve originally supplied the prosomatic appendages of the palseo-
stracan ancestor, while the eye-muscle nerves supplied the body-
muscles of the prosoma.

PROSOMATIC SEGMENTS OF LIMULUS 247

As these appendages did not carry any vital organs such as
branchiae, hut were mainly locomotor and masticatory in function, it
follows that their disappearance as such would be much more com-
plete than that of the mesosomatic branchial appendages. Most
probably, then, in the higher vertebrates no trace of such appendages
might be left ; consequently the segmentation due to their presence
would be very obscure, so that in this region the very reverse of what
is found in the reoion of the vagus nerve would be the rule. There
branchiomeric segmentation is especially evident, owing to the per-
sistence of the branchial part of the branchial appendages; here,
owing to the disappearance of the appendages, the segmentation is
no longer branchiomeric, but essentially mesomeric in consequence
of the persistence of the somatic eye-muscles.

In addition to the evidence of the appendages themselves, the
number of prosomatic segments is well marked out in all the
members of the scorpion group by the divisions of the central
nervous system into well-defined neuromeres in accordance with the
appendages, a segmentation the reminiscence of which may still
persist after the appendages themselves have dwindled or disappeared.
In accordance with this possibility we see that one of the most
recent discoveries in favour of a number of segments in the head-
region of the vertebrate is the discovery in the early embryo of a
number of partial divisions in the brain-mass, forming a system of
cephalic neuromeres which may well be the rudiments of the well-
defined cephalic neuromeres of animals such as the scorpion.

The Evidence of the Peosomatic Musculatuke.

Even if the appendages as such become obscure, yet their muscles
might remain and show evidence of their presence. The most per-
sistent of all the appendage-muscles are the basal muscles which pass
from coxa to carapace and are known by the name of tergo-coxal
muscles. They are large, well marked, segmentally arranged muscles,
dorso- ventral in direction, and, owing to their connecting the limb
with the carapace, are likely to be retained even if the appendage
dwindles away.

The muscular system of Limulus and Scorpio has been investi-
gated by Benham and Miss Beck under Lankester's direction, and the
conclusions to which Lankester comes are these —

248 THE ORIGIN OF VERTEBRATES

The simple musculature of the primitive animal from which hoth
Limulus and the scorpions arose consisted of —

1. A series of paired longitudinal dorsal muscles passing from

tergite to tergite of each successive segment.

2. A similar series of paired longitudinal ventral muscles.

3. A pair of dorso-ventral muscles passing from tergite to

sternite in each segment.

4. A set of dorso-ventral muscles moving the coxa of each limb

in its socket.

5. A pair of veno-pericardial muscles in each segment.

Of these groups of muscles, any one of which would indicate the
number of segments, Groups 1 and 2 do not extend into the proso-
matic region, and Group 5 extends only as far as the heart extends
in the case of hoth Limulus and the Scorpion group ; so that we may
safely conclude that in the Pakeostraca the evidence of somatic
segmentation in the prosomatic region would be given, as far as the
musculature is concerned, by the dorso-ventral somatic muscles
(Group 3), and of segmentation due to the appendages by the
dorso-ventral appendage musculature (Group 4).

Therefore, if, as the evidence so far indicates, the vertebrate has
arisen from a palaeostracan stock, we should expect to find that the
musculature of the somatic segments in the region of the trigeminal
nerve did not resemble the segmental muscles of the spinal region,
was not, therefore, the continuation of the longitudinal musculature
of the body, but was dorso-ventral in position, and that the muscula-
ture of the splanchic segments resembled that of the vagus region,
where, as pointed out in Chapter IV., the respiratory muscles arose
from the dorso-ventral muscles of the mesosomatic appendages.
This is, of course, exactly what is found for the muscles which move
the lateral eyes of the vertebrate ; these muscles, innervated by the
Illrd, IVth, and YIth nerves, afford one of the main evidences of
segmentation in this region, are always grouped in line with the
somatic muscles of spinal segments, and yet cannot be classed as
longitudinal muscles. They are dorso-ventral in direction, and yet
belong to the somatic system ; they are exactly what one ought to
find if they represent Group 3 — the dorso-ventral body-muscles of
the prosomatic segments of the invertebrate ancestor.

The interpretation of these muscles will be given immediately ;
at present I want to pass in review all the different kinds of evidence

PROSOMATIC SEGMENTS OF LIMULUS 249

of segmentation in this region afforded by the examination of the
invertebrate, whether living or fossil, so as to see what clues are left
if the evidence of appendages fails us. I will take in the first instance
the evidence of segmentation afforded by the presence of the muscu-
lature of Group 4, even when, as in the case of many fossils, no
appendages have yet been found. In such animals as Mygale and
Phrynus the prosomatic carapace is seen to be marked out into a
series of elevations and depressions, and upon removing the carapace
we see that these elevations correspond with and are due to the large
tergo-coxal muscles of the appendages ; so that if such carapace alone
were found fossilized we could say with certainty : this animal pos-
sessed prosomatic appendages the number of which can be guessed
with more or less certainty by these indications of segments on the
carapace.

In those forms, then, which are only known to us in the fossil
condition, in which no prosomatic appendages have been found, but
which possess, more or less clearly, radial markings on the prosomatic
carapace resembling those of Phrynus or Mygale, such radial markings
may be interpreted as due to the presence of prosomatic appendages,
which are either entirely concealed by the prosomatic carapace or
dorsal head-plate, or were of such a nature as not to have been
capable of fossilization.

The group of animals in question forms the great group of animals,
chiefly extinct, classified by H. Woodward under the order of Mero-
stomata. They are divided by him into the sub-order of Eurypteridse,
which includes— (1) Pterygotus, (2) Slimonia, (3) Stylonurus, (4)
Eurypterus, (5) Adelophthalmus, (6) Bunodes, (7) Arthropleura, (8)
Hemiaspis, (9) Exapinurus, (10) Pseudoniscus ; and the sub-order
Xiphosura, which includes — (1) Belinurus, (2) Prestwichia, (3)
hamulus.

The evidence of the Xiphosura and of the Hemiaspidaj conclusively
shows, in Woodward's opinion, that the Merostomata are closely
related to the Trilobita, and the Hemiaspida? especially are supposed
to be intermediate between the trilobites and the king-crabs. They
are characterized, as also Belinurus and Prestwichia, by the absence
of any prosomatic appendages, so that in these cases, as is seen in
Fig. 12 (p. 30), representing Bunodes lunula, found in the Eurypterus
layer at Pootzikull, we have an animal somewhat resembling Limulus
in which the prosomatic appendages have either dwindled away and arc

250

THE ORIGIN OF VERTEBRATES

Ce.

Fig. 107. — Phrynus Margme-Maculata.
Cc, median eyes ; lc, lateral eyes ; glab., median plate over brain ; Fo., fovea.

supr.0es.gl--/

Fig. 108. — Phrynus sp. ('?). Cahapace removed.
cam., camerostome ; pi., plastron.

PROSOMATIC SEGMENTS OF LIMULUS

251

completely hidden by the prosomatic carapace, or became so soft as
not to be preserved in the fossilized condition. The appearance of
the prosomatic carapace is, to my
mind, suggestive of the presence of
such appendages, for it is marked out
radially, as is seen in the figure, in a
manner resembling somewhat the mark-
ings on the prosomatic carapace of
Mygale or Phrynus ; the latter mark-
ings, as already mentioned, are due to
the aponeuroses between the tergo-coxal
muscles of the prosomatic appendages
which lie underneath and are attached
to the carapace.

A very similar radial marking is
shown by Woodward in his picture of
Hcmiaspis limuloidcs, reproduced in Fig.
109, found in the Lower Ludlow beds at
Leintwardine. This species has yielded
the most perfect specimens of the genus
Hemiaspis, which is recognized as differ-
ing from Bunodes by the possession of a
telson.

It is striking to find that similar
indications of segments have been found

Fig. 109. — Hemiaspis limuloidcs.
(From Woodward.)

<]L, glabelluui.

on the dorsal surface of the head-region

in many of the must ancient extinct fishes, as will be fully discussed

later on.

The Evidence of Ccelomic Cavities.

In the head-region of the vertebrate, morphologists depend largely
upon the embryonic divisions of the mesoderm for the estimation of
the number of segments, and, therefore, upon the number of ccelomic
cavities in this region, the walls of which give origin to the striated
muscles of the head, so that the question of the number of segments
depends very largely upon the origin of the muscles from the walls of
these head-cavities. It is therefore interesting to examine whether a
similar criterion of segmentation holds good in such a segmented

252 THE ORIGIN OF VERTEBRATES

animal as Limulus, or in the members of the scorpion group, in which

the number of segments are known definitely by the presence of the

appendages. In Limulus we know, from the observations of Kishi-

nouye, that a series of ccelomic cavities are formed embryologically in

the various segments of the mesosoma and prosoma, in a manner

exceedingly similar to their mode of formation in the head-region of

the vertebrate, and he has shown that in the mesosoma a separate

ccelomic cavity exists for each segment, so that just as the dorso-ventral

somatic muscles are regularly segmentally arranged in this region, so

are the ccelomic cavities, and we should be right in our estimation

of the number of segments in this region by the consideration of

the numerical correspondence of these cavities with the mesomatic

appendages. Similarly, in the vertebrate, we find every reason to

believe that a single, separate head-cavity corresponds to each of

the branchial segments in the opisthotic region, and therefore we

should estimate rightly the number of segments by the division of

the mesoderm in this region.

In the prosomatic region of Limulus, the dorso-ventral muscles
are not arranged with such absolute segmental regularity as in the
mesosomatic region, and Kishinonye's observations show that the
ccelomic cavities in this region do not correspond absolutely with
the number of prosomatic appendages. His words are: —

A pair of ccelomic cavities appears in every segment except the
segments of the 2nd, 3rd, and 4th appendages, in which ccelomic
cavities do not appear at all. At least eleven pairs of these cavities
are produced. The eleventh pair belongs to the seventh abdominal
segment.

The first pair of ccelomic cavities is common to the cephalic lobe
and the segment of the first appendage {i.e. the chelicene).

The second ccelomic cavity belongs to the segment of the fifth
appendage. It is well developed.

The ventral portion of the second ccelomic cavity remains as the
coxal gland.

Consequently, if we were to estimate the number of segments in
this region by the number of ccelomic cavities we should not judge
rightly, for we should find only four cavities and seven appendages,
as is seen in the following table : —

rROSOMATIC SEGMENTS of limulus

253

LIMULUS.

VERTEBRATE.

Segments.

Appendages.

Eurypterid appendages.

Ccelomic
cavities.

Ccelomic cavities.

Prosomatic.

1
2

4
5
6

Chelicci'ip or 1st

locomotor.
2nd locomotor
3rd
4th
5th
6th
Chilaria

Cheliccnp

•Endognaths

Ectognath
Metastorna

Premandibular
'Mandibular

0°

a
0

0 ,

•n
0 1

*-H 1

8
9

10
11
12
13
14

Operculum
1st branchial

2nd „

3rd

4th

5th

6th

/-Genital

•Operculum-; 1st bran-
ts chial

2nd branchial

3rd

4th

5th

5
G

>Hyoid

1st branchial
2nd „
3rd „

4th „

The second cavity would in reality represent four segments
belonging to the 2nd, 3rd, 4th, 5th locomotor appendages, i.e. the
very four segments which in the Eurypteridre are concentrated
together to form the endognaths, and we should be justified in put-
ting this interpretation on it, because, according to Kishinouye, its
ventral portion forms the coxal gland, and, according to Lankester, the
coxal gland sends prolongations into the coxa of the 2nd, 3rd, 4th,
5th locomotor appendages. Similarly in the vertebrate, we find three
head-cavities in the region which corresponds, on my theory, to the
prosomatic region of Limulus, (1) the anterior cavity discovered by
Miss Piatt, (2) the premandibular cavity, and (3) the mandibular
cavity, which, if they corresponded with the prosomatic ccelomic cavities
of Limulus, would represent not three segments but seven segments,
as follows: — the anterior cavity would correspond to the first cudomic
cavity, i.e. the cavity of the cheliceral segments in both Limulus and
the Eurypteridie ; the premandibular, to the second ccelomic cavity,
representing, therefore, the 2nd, 3rd, 4th, 5th prosomatic segments in
Limulus and the endognathal segments in the Eurypteridre ; and the
mandibular to the 3rd and 4th ccelomic cavities, representing the last
locomotor and chilarial segments in Limulus, i.e. the ectoguathal and
metastomal segments in the Eurypteridce.

254 THE O RIG IX OF VERTEBRATES

It is worthy of note that, in respect to their ccelomic cavities, as in
the position and origin of their nerves in the central nervous system,
the first pair of appendages, the chelicerre, retain a unique position,
differing from the rest of the prosomatic appendages.

In the table I have shown how the vertebrate crelomic cavities
may be compared with those of Limulus. The next question to con-
sider is the evidence obtained by morphologists and anatomists as to
the number of segments supplied by the trigeminal nerve-group; this
question will be considered in the next chapter.

Summary.

In Chapters IV. and V. I have dealt with the opisthotic segments of the
vertebrate, including" therein the segments supplied by the facial nerve, and
shown that they correspond to the mesosomatic segments of the palaaostracan ;
consequently the facial (VII.). glossopharyngeal (IX.), and vagus (X.) nerves
originally supplied the branchial and opercular appendages.

In this chapter the consideration of the pro-otic segments is commenced,
that is. the seg-nients supplied by the trigeminal (V.) and the eye-muscle nerves
(III.. IV., VI.). I have considered the Vlth nerve with the rest of the eye-
muscle nerves for convenience' sake, though in reality it belongs to the same
segment as the facial. Of these, that part of the trigeminal which innervates
the muscles of mastication corresponds to the splanchnic seg-ments, while the
eye-muscle nerves belong* to the corresponding- somatic seg-ments ; but the
pro-otic segments of the vertebrate ought to correspond to the prosomatic
segments of the invertebrate, just as the oinsthotic correspond to the meso-
somatic. Therefore the motor part of the trigeminal ought to supply muscles
which orig-inally moved the prosomatic appendages, while the eye-muscles ought
to have belonged to the somatic part of the same segments.

The first question considered is the number of segments which ought to be
found in this region. In Limulus. the Eurypterida?. and the scorpions there are
seven prosomatic segments which carry (1) the chelicerae, (2, 3. 4, 5) the four
first locomotor appendages — the endognaths. (6) the large special appendage —
the ectognath — and (7) the appendag-es, which in Limulus are known as the
chilaria, and are small and insig-nificant, but in Eurypterus and other forms
grow forwards, fuse together, and form a single median lip to an accessory oral
chamber, which lip is known as the metastoma. Of these appendages the
chnlicerae and endognaths tend to dwindle away and become mere tentacles,
while the large swimming- ectognath and metastoma remain strong- and
vigorous.

In this, the prosomatic region, the somatic segmentation is not characterized
by the presence of the longitudinal muscle segments, for they do not extend
into this head-region, but only by the presence of the seg-mental somatic ventro-

PROSOMATIC SEGMENTS OF LIMULUS 255

dorsal muscles. Among the muscles of the appendages the system of large
tergo-coxal muscles is especially apparent.

From these considerations it follows that the number of segments in this
region in the vertebrate ought to be seven; that the musculature supplied by
the trigeminal nerve ought to represent seven ventral or splanchnic segments,
of which only the last two are likely to be conspicuous ; and that the musculature
supplied by the eye-muscle nerves ought to be dorso-ventral in direction, which
it is, and represent seven dorsal or somatic segments.

A further peculiarity of this region, both in Limuhis and the scorpions,
is found in the excretory organs which are known by the name of coxal glands,
becaiise they extend into the basal joint, or coxa, of cei'tain of the prosomatic limbs.
The appendages so characterized are always the four endognaths. and it follows
that if these four endognaths lose their locomotor power, become reduced in
size, and concentrated together to form mere tentacles, then of necessity the
coxal gdands will be concentrated together, and tend to form a glandular mass
in the region of the mouth ; in fact, take up a position corresponding to that
of the pituitary body in vertebrates.

Taking all these facts into consideration, it is possible to construct a drawing
of a sagittal section through the head-region of Eurypterus, which will
represent, with considerable probability, the arrangement of parts in that
animal. This can be compared with the corresponding section through the
head of Ammocoetes.

Now, as pointed out in the last chapter, the early stage of Ammocoetes is
remarkably different from the more advanced stage ; at that time the septum
between the oral and respiratory chambers has not yet broken through, and the
olfactory or nasal tube, known at this stage as the tube of the hypophysis, is
directed ventrally, not dorsally.

The comparison of the diagram of Eurypterus with that of the early stage
of Ammocoetes is remarkably close, and immediately suggests not only that the
single nose of the former is derived from the corresponding organ in the
pakeostracan, but that the pituitary body is derived from the concentrated
coxal gdands, and the lower lip from the metastoma. The further working out
of these homologies will be discussed in the next chapter.

In addition to the evidence of segmentation afforded by the appendag-es, there
are in this region, in Limulus and the scorpion g*roup. three other criteria of
segmentation available to us, if from any cause the evidence of appendages fails
us. These are —

1. The number of neuromeres are marked out in this region of the brain
more or less plainly, especially in the young animal, just as they are also in
the embryo of the vertebrate.

2. The segmentation is represented here, just as in the mesosomatic region,
by two sets of muscle-segments ; the one somatic, consisting of the seg-mentally
arranged dorso-ventral muscles, the continuation of the group already discussed
in connection with the mesosomatic segmentation, and the other appendicular
characterized by the tergo-coxal muscles. These latter segmental muscles are
especially valuable, for in such forms as Mygale, Phrynus, etc., their presence
is indicated extei-nally by marking's on the prosomatic carapace, and thus corre-
sponding markings found on fossil carapaces or on dorsal head-shields can be

256 THE ORIGIN OF VERTEBRATES

interpreted. These two sets of muscle-segments correspond in the vertebrate
to the somatic and splanchnic segmentations.

3. In the vertebrate the segmentation in this region is indicated by the
ccelomic or head-cavities, which are cavities formed in the mesoderm of the
embryo, the walls of which give origin to the striated muscles of the head. In
Limulus corresponding ccelomic cavities are found, which are directly comparable
with those found in the vertebrate.

CHAPTEE VIII

THE SEGMENTS BELONGING TO THE TRIGEMINAL

NERVE-GROUP

The prosomatic segments of the vertebrate. — Number of segments belonging
to the trigeminal nerve-group. — History of cranial segments. — Eye-muscles
and their nerves.— Comparison with the dorso- ventral somatic muscles of the
scorpion. — Explanation of the oculomotor nerve and its group of muscles.
— Explanation of the trochlearis nerve and its dorsal crossing. — Explana-
tion of the abducens nerve. — Number of segments supplied by the
trigeminal nerves. — Evidence of their motor nuclei. — Evidence of their
sensory ganglia. — Summary.

From the evidence given in the last chapter, combined with that
given in Chapter IV., the probability of the theory that the trigeminal
group of nerves of the vertebrate have been derived from the
prosomatic group of nerves of the invertebrate can be put to the
test by the answers to the following morphological and anatomical
questions : —

1. Do we find in the vertebrate two segmentations in this region
corresponding to the two segmentations in the branchial region, i.e.
a somatic or dorsal series of segments, and a splanchnic or ventral
series of segments ? The latter would not be branchial, but rather
of the nature of free tactile appendages ; so that it is useless to look
for or talk about gill-slits, although such appendages, being serially
homologous with the branchial mesosomatic appendages, would
readily give rise to the conception of branchial segments.

2. Is there morphological evidence that the trigeminal nerve is
not the nerve belonging to a single segment, or even to two segments,
but is really a concentration of at least six, probably seven, segmental
nerves ?

3. Is there morphological evidence that the oculomotor and
trochlear nerves, which on all sides are regarded as belonging to
the trigeminal segments, are not single nerves corresponding each

258 THE ORIGIN OF VERTEBRATES

to a single segment, but are the somatic motor roots belonging to
the same segments as those to which the trigeminal supplies the
splanchnic roots ?

4. Do the mesoderm segments, which give origin to the eye-
muscles, and therefore do the head-cavities of this region, correspond
with the trigeminal segments 1 Considering the concentration of
parts in this region and the difficulty already presented by the want
of numerical agreement between the prosomatic appendages and the
prosomatic ccelomic cavities in Limulus, it may very probably be
difficult to determine the actual number of the mesoderm segments.

5. Is there anatomical evidence that the ganglion of origin of the
motor part of the trigeminal nerve is not a single ganglion, but a
representative of many, probably seven ?

6. Is there anatomical evidence that the ganglia of origin of the
oculomotor and trochlear nerves represent many ganglia ?

7. Is there any evidence that the organs originally supplied by
the motor part of the trigeminal nerve are directly comparable with
prosomatic appendages ?

It is agreed on all sides that in this region of the head there is
distinct evidence of double segmentation, the dorsal mesoderm segments
giving origin to the eye-muscles, and the ventral segments to the
musculature innervated by the trigeminal nerve. Originally, accord-
ing to the scheme of van Wijhe, two segments only were recognized,
the dorsal parts of which were innervated by the Illrd and IVth
nerves respectively. Since his paper, the tendency has been to
increase the number of segments in this region, as is seen in the
following sketch, taken from Eabl, of the history of cranial
segmentation.

HlSTOEY OF CRANIAL SEGMENTATION.

The first attempt to deal with this question was made by Goethe
and Oken. They considered that the cranial skeleton was composed
of a series of vertebrae, but as early as 1842 Vogt pointed out that
only the occipital segments could be reduced to vertebrae. In 1869,
Huxley showed that vertebrae were insufficient to explain the
cranial segmentation, and that the nerves must be specially con-
sidered. The olfactory and optic nerves he regarded as parts of
the brain, not true segmental nerves ; the rest of the cranial nerves

SEGMENTS OF TRIGEMINAL NERVE-GROUP 259

were segmental, with special reference to branchial arches and clefts,
the facial, glossopharyngeal, and separate vagus branches supplying
the walls of the various branchial pouches. In a similar manner,
the supra- and infra-maxillary branches of the trigeminal were
arranged on each side of the mouth, and the inner and outer twigs of
the first (ophthalmic) branch of the trigeminal on each side of the
orbito-nasal cleft, the trabecular and the supra-maxillary arches being
those on each side of this cleft. Thus Huxley considered that there
was evidence of a series of pairs of ventral arches belonging to the
skull, viz. the trabecular and maxillary in front of the mouth, the
mandibular, hyoid, and branchial arches behind, and that the Vth,
Vllth, IXth, and Xth nerves were segmental in relation to these
arches and clefts. Gegenbaur, in 1871 and 1872, considered that the
branchial arches represented the lower arches of cranial vertebrae,
and therefore corresponded to lower arches in the spinal region,
i.e. the skull was composed of as many vertebrae as there are
branchial arches. These vertebrae were confined to the notochordal
part of the skull, the prechordal part having arisen secondarily from
the vertebral part, while the number of vertebrae are at least nine,
possibly more. The nerves which could be homologized with spinal
nerves were, he thought, divisible into two great groups— (1) the
trigeminal group, which included the eye-muscle nerves, the facial,
and its dorsal branch, the auditory; (2) the vagus group, which
included the glossopharyngeal and vagus.

Such was the outcome of the purely comparative anatomical
work of Huxley and Gegenbaur — work that has profoundly influenced
all the views of segmentation up to the present day.

Now came the investigations of the embryologists, of whom I
will take, in the first instance, Balfour, whose observations on the
embryology of the Selachians led him to the conclusion that besides
the evidence of segmentation to be found in the cranial nerves and
in the branchial clefts, further evidence was afforded by the existence
of head-cavities, the walls of which formed muscles just as they do
in the spinal region. He came to the conclusion that the first head-
cavity belonged to one or more pre-oral segments, of which the nerves
were the oculomotor, trochlearis, and possibly abducens ; while there
were seven post-oral segments, each with its head-cavity and its
visceral arch, of which the trigeminal, facial, glossopharyngeal, and
the four parts of the vagus were the respective nerves.

260 THE ORIGIN OF VERTEBRATES

Marshall; iu 1882, considered that the cranial segments were all
originally respiratory, and that all the segmental nerves are arranged
uniformly with respect to a series of gill-clefts which have hecome
modified anteriorly and have heen lost, to a certain extent, pos-
teriorly. He included the olfactory nerves among the segmental
nerves, and looked upon the olfactory pit, the orbito-nasal lacrymal
duct, the mouth, and the spiracle as all modified gill-slits, so that he
reckoned three pre-oral and oral segments belonging to the 1st, Illrd,
IVth, and Yth nerves, and eight post-oral segments belonging respec-
tively to the Vllth and Vlth nerves, and to the IXth nerve, and six
segments belonging to the Xth nerve. He pointed out that muscles
supplied by the oculomotor nerve develop from the outer wall of the
first head-cavity ; not, however, the dbliguus superior and rectus
cxtcmus, the latter originating probably from the walls of the third
cavity.

In the same year, 1882, came van Wijhe's well-known paper, in
which he showed that the mesoderm of the head in the selachian
divided into two sets of segments, dorsal and ventral ; that the dorsal
segments were continuous with the body-somites, and that the ven-
tral segments formed the lateral plates of mesoblast between each of
the visceral and branchial pouches. He concluded that the dorsal
somites were originally nine in number, that each was supplied with
a ventral nerve-root, in the same way as the somites in the trunk,
and that to each one a visceral pouch corresponded, whose walls
were supplied by the corresponding dorsal nerve-root ; of these nine
segments, the ventral nerve-roots of the first three segments were
respectively the oculomotor, trochlearis, and abducens nerves. The
next three segments possessed no definable ventral root or muscles,
and the seventh, eighth, and ninth segments possessed as ventral
roots the hypoglossal nerve, with its muscular supply. The corre-
sponding dorsal nerve-roots were the trigeminal, facial, auditory,
glossopharyngeal and vagus nerves, the difference between cranial
and spinal dorsal roots beiug that the former contain motor
fibres.

Ahlborn, in 1884, drew a sharp distinction between the segments
of the mesoderm and those of the endoderm. The former segmenta-
tion he called mesomeric, the latter branchiomeric. He considered
the two segmentations to be independent, and concluded that the
branchiomeric was secondary to the mesomeric, and therefore not of

SEGMENTS OF TRIGEMINAL NERVE-GROUP 26 1

segmental value. As to the segments of the mesoderm in the head,
the three hindmost or occipital in Petroniyzontidfle remain perma-
nently, and correspond to the three last segments in the selachian head-.
Of the anterior mesoderm segments, he considered that there were
originally six, and that there are six typical eye-muscles in all
Craniota, which have been compressed into three segments, as in
Selachia.

Froriep (1885) showed in sheep-embryos and in chicks that the
hypoglossal nerve belongs to three proto-vertebrre posterior to the
vagus region, which were true spinal segments. He therefore modified
Gegenbaur's conceptions to this extent : that portion of the skull
designated by Gegenbaur as vertebral must be divided into two parts
— a hind or occipital region, which is clearly composed of modified
vertebras and is the region of the hypoglossal nerves, and a front
region, extending from the oculomotor to the accessorius nerves, which
is characterized segmentally by the formation of branchial arches, but
in which there is no evidence that proto-vertebree were ever formed.
He therefore divides the head-skeleton into three parts —

1. Gegenbaur's e vertebral part — the region of the olfactory and
optic nerves — which cannot be referred to any metameric segmen-
tation.

2. The pseudo-vertebral, pre-spinal, or branchial part, clearly
shown to be segmented from the consideration of the nerves and
branchial arches, but not referable to proto-vertebraa — the region of
the trigeminal and vagus nerves.

3. The vertebral spinal part — the region of the hypoglossal
nerves.

He further showed that the ganglia of the specially branchial
nerves, the facial, glossopharyngeal, and vagus, are at one stage
in connection with the epidermis, so that these parts of the epidermis
represent sense-organs which do not develop ; these organs probably
belonged to the lateral line system. As the connection takes place
at the dorsal edge of the gill- slits, they may also be called rudimen-
tary branchial sense-organs.

Since this paper of Froriep's, it has been generally recognized,
and Gegenbaur has accepted Froriep's view, that the three hindmost
metameres, which distinctly show the characteristics of vertebras,
belong to the spinal and not to the cranial region, so that the
metameric segmentation of the cranial region proper has become

262 THE ORIGIN OF VERTEBRATES

more and more associated with the branchial segmentation. Froriep's
discovery of the rudimentary branchial sense-organs as a factor in
the segmentation question has led Beard to the conclusion that the
olfactory and auditory orgaus represent in a permanent form two
of these rudimentary branchial sense-organs. He therefore includes
both the olfactory and auditory nerves in his list of cranial segmental
nerves, and makes eleven cranial branchial segments in front of the
spinal segments represented by the hypoglossal.

A still larger number of cranial segments is supposed to exist,
according to the researches of Dohrn and Killian, in the embryos
of Torpedo ocellata. The former, holding to the view that vertebrates
arose from annelids, considered that the head was formed of a series
of metameres, to each one of which a mesoderm-segment, a gill-arch,
a gill-cleft, a segmental nerve and vessel belonged. He found in the
front head-region of a Torpedo embryo, corresponding to van Wijhe's
first four somites, no less than twelve to fifteen mesoderm segments,
and concluded, therefore, that the eye-muscle nerves, especially the
oculomotor, represented many segmental nerves, and were not the
nerves of single segments ; so, also, that the inferior maxillary part of
the trigeminal and the hyoid nerve of the facial are probably not
single nerves, but a fusion of several. Killian comes to much the
same conclusion as Dohrn, for he finds seventeen to eighteen separate
mesoderm segments in the head, of which twelve belong to the tri-
geminal and facial region.

Since Eabl's paper, a number of papers have appeared, especially
from America, dealing with yet another criterion of the original
segmentation of the head, viz. a series of divisions of the central
nervous system itself, which are seen at a very early stage of
development, and are called neuromeres ; the divisions in the cranial
region being known as encephalomeres, and those of the spinal region
as myomeres. Locy's paper has especially brought these divisions
into prominence as a factor in the question of segmentation. They
are essentially segments of the epiblast and not of the mesoblast ;
they are conspicuous in very early stages, and appear to be in
relation with the cranial nerves, according to Locy. He recognizes
in Squalus acanthias, in front of the spino-occipital region, fourteen
pairs of such encephalomeres and a median unsegmented termination,
which may represent one more pair fused in the middle line, making
at least fifteen. He distributes these fifteen segments as follows :

SEGMENTS OE TRIGEMINAL NERVE-GROUP 263

fore-brain three and unsegmented termination, mid-brain two, and
hind-brain nine.

Again, Knpffer, in his recent papers on the embryology of Ammo-
ccetes, asserts that especial information as to the number of primitive
segments is afforded by the appearance in the early stages of a series
of epibranchial ganglia in connection with the cranial nerves, which
remain permanently in the case of the vagus nerves, but disappear
in the case of pro-otic nerves. He considers that the evidence points
to the number of segments in the mid- and hind-brain region as
being primitively fifteen, viz. six segments belonging to the tri-
geminal and abducens group, three segments belonging respectively
to the facial, auditory, and glossopharyngeal, and six to the vagus.

From this sketch we see that the modern tendency is to make six
segments at least out of the region of the trigeminal nerves rather
than two. In this region, as already mentioned, the evidence of
segmentation is based more clearly on the somatic than on the
splanchnic segments. We ought, therefore, in the first place, to
consider the teaching of the eye-muscles and their nerves and the
ccelomic cavities in connection with them, and see whether the
hypothesis that such muscles represent the original dorso-ventral
somatic muscles of the pakeGstracan ancestor is in harmony with
and explains the facts of modern research.

Eye-Muscles and their Nerves.

The only universally recognized somatic nerves belonging to these
segments which exist in the adult are the nerves to the eye-muscles,
of which, according to van Wijhe, the oculomotor is the nerve of the
1st segment, the trochlearis of the 2nd, and the abducens of the 3rd ;
while the nerves and muscles belonging to the 4th and 5th segments,
i.e. the 2nd facial and glossopharyngeal segments respectively, show
only the merest rudiments, and do not exist in the adult. One
significant fact appears in this statement of van Wijhe, and is
accepted by all those who follow him, viz. that the oculomotor nerve
has equal segmental value with the trochlearis and the abducens,
although it supplies a number of muscles, each of which, on the face
of it, has the same anatomical value as the superior oblique or
external rectus. Dohrn alone, as far as I know, as already pointed
out, insists upon the multiple character of the oculomotor nerve.

264 THE 0 RIG IX OF VERTEBRATES

As far as the anatomist is concerned, the evidence is becoming
clearer and clearer that the nucleus of the Illrd nerve is a composite
ganglion composed of a number of nuclei, each similar to that of the
trochlearis, so that if the trochlearis nucleus is a segmental motor
nucleus, then the oculomotor nucleus is a combined nucleus belong-
ing to at least four segmental nerves, each of which has the same
value as that of the trochlearis.

The investigations of a number of anatomists, among whom may
be mentioned Gudden, Obersteiner, Edinger, Kolliker, Gehuchten,
all lead directly to the conclusion that this oculomotor nucleus is
composed of a number of separate nuclei, of which the most anterior
as also the Edinger- Westphal nucleus contains small cells, while the
others contain lar^e cells. Thus Edinger divides the origin of the
oculomotor nerve into a small-celled anterior part and a larger
posterior part, of which the cells are larger and distinctly arranged
in three groups — (1) dorsal, (2) ventral, and (3) median. Between
the anterior and posterior groups lies the Edinger- Westphal nucleus,
which is small-celled ; naturally, the large-celled group is that which
gives origin to the motor nerves of the eye-muscles, the small-celled
being possibly concerned with the motor nerves of the pupillary and
ciliary muscles. I may mention that Kolliker considers that the
anterior lateral nucleus has nothing to do with the oculomotor nerve,
but is a group of cells in which the fibres of the posterior longi-
tudinal bundle and of the deep part of the posterior commissure
terminate.

These conclusions of Edinger are the outcome of work done in
bis laboratory by Perlia, who says that in new-born animals the
nucleus of origin of the oculomotor nerve is made up of a number
of groups quite distinct from each other, each group being of the
same character as that of the trochlearis. He finds the same
arrangement in various mammals and birds. Further, he finds that
some of the fibres arise from the nucleus of the opposite side, thus
crossing, as in the trochlearis ; these crossing fibres belong to the
most posterior of the dorsal group of nuclei, i.e. to the nerve to the
inferior oblique muscle.

The evidence, therefore, points to the conclusion that the oculo-
motor nucleus is a multiple nucleus, each part of which gives origin
to one of the nerves of one of the eye-muscles.

Edinger says that such an array of clinical observations exists,

SEGMENTS OF TRIGEMINAL NERVE-GROUP 265

and of facts derived from post-mortem dissections, that one may
venture to designate the portion of the nucleus from which the
innervation of each individual ocular muscle comes. He gives Starr's
table, the latest of these numerous attempts, begun by Pick. Accord-
ing to Starr, the nuclei of the nerves to the individual muscles are
arranged from before backward, thus —

m. sphincter iridis. m. ciliaris.

m. levator palpebral, m. rectus intemus.

m. rectus superior. m. rectus inferior.

m. ohliquus inferior.
Further, the evidence of the well-known physiological experi-
ments of Hensen and Volckers that the terminal branches of the
oculomotor nerve arise from a series of segments of the nucleus,
arranged more or less one behind the other in a longitudinal row,
leads them to the conclusion that the nuclei of origin are arranged as
follows, proceeding from head to tail : —

Nearest brain.

on. ciliaris.

m. sphincter iridis.

m. rectus intemus.

m. rectus superior.

m. levator palpebral.

m. rectus inferior.

Most posterior.

m. ohliquus inferior.

It is instructive to compare this arrangement of Hensen and
Volckers with the arrangement of the origin of these muscles from
the premandibular cavity as given by Miss Piatt.

Thus she states that the most posterior part of the premandibular
cavity is cut off so as to form a separate cavity, resembling, except
in position, the anterior cavity ; this separate, most posterior part
gives origin to the inferior oblique muscle. She then goes on to
describe how the dorsal wall of the remainder of the premandibular
cavity becomes thickened, to form posteriorly the rudiment of the
inferior rectus and anteriorly the rudiments of the superior and
internal recti, a slight depression in the wall of the cavity separating
these rudiments. The internal rectus is the more median of the
two anterior muscles. In other words, her evidence points not only
to a fusion of somites to form the premandibular cavity, but also
to the arrangement of these somites as follows, from head to tail :
(1) internal rectus, (2) superior rectus, (3) inferior rectus, (4) inferior

266 THE ORIGIN OF VERTEBRATES

oblique — an order precisely the same as that of Hensen and Volckers,
and of Starr.

I conclude, from the agreement between the anatomical, physio-
logical, and morphological evidence, that the Illrd and IVth nerves
contain the motor somatic nerves belonging to the same segments as
the motor trigeminal, in other words, to the prosomatic segments, so
that the eye-muscles, innervated by III. and IV., represent segmental
muscles belonging to the prosoma. Further, I conclude that originally
there were seven prosomatic segments, the first of which is repre-
sented by the anterior cavity described by Miss Piatt, and does not
form any permanent muscles ; that the next four belong to the pre-
mandibular cavity, and the muscles formed are the superior rectus,
internal rectus, inferior rectus, and inferior oblique ; and that the last
two belong to the mandibular cavity, the muscles formed being Miss
Piatt's mandibular muscle and the superior oblique. It is, to say the
least of it, a striking coincidence that such an arrangement of the
ccelomic cavities as here given should be so closely mimicked by
the arrangement in the prosomatic region of Limulus as already
mentioned ; it suggests inevitably that the head-cavities of the verte-
brate are nothing more than the prosomatic and mesosomatic
segmental ccelomic cavities, as found in animals such as Limulus.
In the table on p. 253, 1 have inserted the segments in the vertebrate
for comparison with those of Limulus.

Before we can come to any conclusion as to the original position
of these eye-muscles, it is necessary to consider the Vlth nerve and
the external rectus muscle. This nerve and this muscle belong to
van Wijhe's 4th segment. The muscle is, therefore, the somatic
segmental muscle belonging to the same segment as the facial and is,
in fact, a segmental muscle belonging not to the prosoma, but to the
mesosoma. Neal comes to the conclusion that the existing abducens
is the only root which remains permanent among a whole series of
corresponding ventral roots belonging to the opisthotic segments, and
further points out that the external rectus was originally an opis-
thotic muscle which has taken up a pro-otic position, or, translating
this statement into the language of Limulus, etc., it is a mesosomatic
muscle which has taken up a prosomatic position.

There is, however, another muscle — the Retractor oculi — belonging
to the same group which is innervated by the Vlth nerve. Quite
recently Edgeworth lias shown that in birds and reptiles this muscle

SEGMENTS OF TRIGEMINAL NERVE-GROUP 267

belongs to the hyoid segment ; so that in. this respect also the hyoid
segment proclaims its double nature.

With respect to the external rectus muscle, Miss Piatt has shown
that the mandibular muscle is formed close alongside the external rectus,
so that the two are in close relationship as long as the former exists.

Further, as already mentioned, the eye-muscles in Ammoccetes
must be considered by themselves ; they do not belong in structure
or position to the longitudinal somatic muscles innervated by the
spinal nerves ; their structure is not the same as that of the tubular
constrictor or branchial muscles, but resembles that structure some-
what ; their position is dorso-ventral rather than longitudinal ; they
may be looked upon as a primitive type of somatic muscles seg-
mentary arranged, the direction of which was dorso-ventral.

Anderson also has shown that the time of medullation of the
nerves supplying these muscles is much earlier than that of the
nerves belonging to the somatic trunk-muscles, their medullation
taking place at the same time as that of the motor nerves supplying
the striated visceral muscles ; and Sherrington has observed that
these muscles do not possess muscle-spindles, while all somatic
trunk-muscles do. Both these observations are strong confirmation
of the view that the eye-muscles must be classified in a different
category to the ordinary somatic trunk muscle group.

What, then, is the interpretation of these various embryological
and anatomical facts ?

Eemembering the tripartite division of each segmental nerve-group
in Limulus into (1) dorsal or sensory somatic nerve, (2) appendage-
nerve, and (3) ventral somatic nerve, I venture to suggest that the
three nerves — the oculomotor ius, the trochlearis, and the abducens
— represent the ventral somatic nerves of the prosoma, and partly
also of the mesosoma ; that they are nerves, therefore, which may
have originally contained sensory fibres, and which still contain the
sensory fibres of the eye-muscles themselves, as stated by Sherrington.
According to this suggestion, the eye-muscles are the sole survivors
of the segmental dorso-ventral somatic muscles, so characteristic of
the group from which I imagine the vertebrates to have sprung. In
the mesosomatic region the dorso-ventral muscles which were retained
were those of the appendages and not of the mesosoma itself, because
the presumed ancestor breathed after the fashion of the water-
breathing Limulus, by means of the dorso-ventral muscles of its

268 THE ORIGIN OF VERTEBRATES

branchial appendages, and not after the fashion of the air-breathing
scorpion, by means of the dorso-ventral muscles of the mesosoma.
The only mesosomatic dorso-ventral muscles which were retained
were those of the foremost mesosomatic segments, i.e. those supplied by
the Vlth nerve, which were preserved owing to their having taken on
a prosomatic position and become utilized to assist in the movements
of the lateral eyes.

Let us turn now to the consideration of the corresponding muscu-
lature in Limulus and in the scorpion group. These muscles con-
stitute the markedly segmental muscles to which I have given the
name ' dorso-ventral somatic muscles.' They are most markedly
segmental in the mesosomatic region, both in Limulus and in Scorpio,
each mesosomatic segment possessing a single pair of these vertical
mesosomatic muscles, as Benham calls them (cf. Fig. 58 {Dv.)). In
the prosomatic region the corresponding muscles are not so clearly
defined in Limulus; they are apparently attached to the plastron
forming the group of plastro-tergal muscles. From Benham's descrip-
tion it is sufficiently evident that they formed originally a single pair
to each prosomatic segment.

In Scorpio, according to Miss Beck, the dorso-ventral prosomatic
muscles are situated near the middle line on each side and form the
following well-marked series of pairs of muscles, shown in Fig. 110, A,
taken from her paper, and thus described by her : —

1. The dorso-cheliceral-sternal muscle (61) is the most anterior
of the dorso-ventral muscles. It is very small, and is attached to the
carapace near the median line anteriorly to the central eyes.

2. The median dorso-preoral-entosclerite muscle (62) is a large
muscle, between which and its fellow of the opposite side the eyes are
situated. It is attached dorsally to the carapace and ventrally to the
pre-oral entosclerite.

3. The anterior dorso-plastron muscle (03) is attached dorsally
to the carapace in the middle line, being joined to its fellow of the
opposite side. They separate, and are attached ventrally to the
plastron. Through the arch thus formed the alimentary canal and
the dorsal vessel pass.

4. The median dorso-plastron muscle (64) is attached dorsally to
the posterior part of the carapace. It runs forward on the anterior
surface of the posterior flap of the plastron to the body of the plastron,
to which it is attached.

SEGMENTS OF TRIGEMINAL NERVE-GROUP 269

DOESO - VENTRAL MUSCLES ON

Carapace of Scorpion. (From
Miss Beck.)

Similar Muscles on Carapace
of Eurypterus.

Similar Muscles on Head-
Shield OF A CEPHALASPID.

I.e., lateral eyes ; c.e., central
eyes ; Fro., narial opening.

62-65 refer to Miss Beck's cata-
logue of the scorpion muscles.

Fig. 110.

270 THE ORIGIN OF VERTEBRATES

To these may be added, owing to its attachment to the plastron,

5. The posterior dorso-plastron muscle (65). This is the first of
the dorso-ventral muscles attached to the mesosomatic tergites, being
attached to the tergite of the first segment of the mesosoma.

This muscle is of interest, in connection with the prosomatic
dorso-ventral muscles, because it is attached to the plastron, and runs
a course in close contact with the muscle (64), the two muscles being
attached dorsally close together, on each side of the middle line, the
one at the very posterior edge of the prosomatic carapace, and the
other at the very anterior edge of the mesosomatic carapace.

Taking these muscles separately into consideration, it may be
remarked with respect to (61) that the cheliceral segment in its
paired dorso-ventral muscles, as in its tergo-coxal muscles, takes
up a separate position isolated from the rest of the prosomatic
segments.

Next comes (62) the median dorso-preoral-entosclerite muscle,
which is strikingly different from all the other dorso-ventral muscles
in its large size and the extent of its attachment to the dorsal cara-
pace, according to Miss Beck's figures. The reason of its large size
is clearly seen upon dissection of the muscles in Buthus, for I find
that, strictly speaking, it is not a single muscle, but is composed of
a series of muscle-bundles, separated from each other by connective
tissue. There are certainly three separate muscles included in this
large muscle, which are attached in a distinct series along the pre- oral
entosclerite, and present the appearance given in Fig. 110, A, at their
attachment to the prosomatic carapace. Of this muscle-group the
most anterior and the most posterior bundle are distinctly separate
muscles ; I am not, however, clear whether the middle bundle
represents one or two muscles.

This division of Miss Beck's muscle (62) into three or four
muscles brings the prosomatic region of the scorpion into line with
the mesosomatic, and enables us to feel sure that a single pair of
dorso-ventral somatic muscles belongs to each prosomatic segment
just as to each mesosomatic, and, conversely, that each such single
pair of muscles possesses segmental value in this region as much as
in the mesosomatic.

It is very striking to see how in all the Scorpionida3, in which the
two median eyes are the principal eyes, this muscle group (62) on
the two sides closely surrounds these two eyes, so that with a fixed

SEGMENTS OF TRIGEMINAL NERVE-GROUP 271

pry-oral eutosclerite, a slight movement of the eyes, laterally or
anteriorly, owing to the flexibility of the carapace, might result as
the consequence of their contraction. But this cannot be the main
object of these muscles. The pre-oral entosclerite is firmly fixed to
the camerostome, as is seen in Fig. 94, pr. ent., so that the main
object of these muscles is, as Huxley has pointed out, the movement
of this organ.

In order to avoid repetition of the long name given to this muscle
group (62) by Miss Beck, because of their position, and for other
reasons which will appear in the sequel, I will call this group of
muscles the group of recti muscles. These recti muscles belong
clearly to the segments posterior to the first prosomatic or cheliceral
segment, and represent certainly three, probably four, of these
segments, i.e. belong to the segments corresponding to the second,
third, fourth, and fifth prosomatic locomotor appendages — the endo-
gnaths of the old Eurypterids.

The next pair of muscles is the pair of anterior dorso-plastron
muscles (63). This muscle-pair evidently belongs to a segment pos-
terior to the segments represented by the group already discussed,
and belongs, therefore, in all probability to the same segment as the
sixth pair of prosomatic appendages — the ectognaths of the old
Eurypterids. This can be settled by considering either the nerve-
supply or the embryological development. In the Eurypterids it
seems most highly probable that the dorso- ventral muscles of each
half of the segments belonging to the endognaths should be compressed
together and separate from the dorso-ventral muscle belonging to the
ectognathal segment, on account of the evident concentration and small
size of the endognathal segments in contradistinction to the separate-
ness and large size of the ectognathal segment.

The striking peculiarity of this muscle-pair, which distinguishes it
from all other muscles in the scorpion, is the common attachment of
the muscles of the two sides in the mid-dorsal line, so that the pair
of muscles forms an arch through which the alimentary canal and
dorsal blood-vessel pass.

The same dorso-ventral muscles are present in Phrynus, and in
this animal the fibres of this pair of muscles (63) actually interlace
before the attachment to the prosomatic carapace, so that the attach-
ment of the muscle on each side overpasses the mid-dorsal line, and
a true crossing occurs. In Fig. 108 the position of this pair of

272 THE ORIGIN OF VERTEBRATES

muscles is shown just posteriorly to the brain-mass. This muscle
I will call the oblique muscle.

Finally we come to the muscles (64) and (65), the median and
posterior dorso-plastron muscles, which run close together. Both
muscles are attached to the plastron, and, therefore, to that extent
belong to the prosomatic region ; they are attached dorsally close to
the junction of the prosoma and mesosoma. This position of the
first mesosomatic dorso-ventral muscle belonging to the opercular
segment may be compared with the position of the first mesosomatic
dorso-ventral muscle in Limulus which has become attached to the
prosomatic carapace ; in both cases we see an indication that the
foremost pair of mesosomatic dorso-ventral somatic muscles tend to
take up a prosomatic position.

As to the pair of small muscles (64), I believe that they repre-
sent the dorso-ventral muscles of the seventh prosomatic segment
(if the pair of muscles (63) belongs to the segment of the sixth loco-
motor prosomatic appendages), i.e. they belong to the chilarial
segment or metastoma.

I desire to draw especial attention to the fact that the dorso-
ventral muscle (64), which represents the seventh segment, always
runs close alongside the dorso-ventral muscle (65), which represents
the first mesosomatic or opercular segment.

The comparison, then, of these two sets of facts leads to the
following conclusions : —

The foremost prosomatic or trigeminal segment stood separate
and apart, being situated most anteriorly ; the musculature of this
segment does not develop, so that the only evidence of its presence
is given by the anterior ccelomic cavity. This corresponds, according
to my scheme, with the first or anterior coelomic cavity of Limulus,
and therefore represents, as far as the prosomatic appendages are
concerned, the first prosomatic appendage-pair, or the chelicene ; the
appendage-muscles being the muscles of the chelicerse, and the
dorso-ventral somatic muscles the pair of dorso-cheliceral sternal
muscles (61) in the scorpion. Both these sets of muscles, therefore,
dwindle and disappear in the vertebrate.

Then came four segments fused together to form the preman-
dibular segment, the characteristic of which is the apparent non-
formation of any permanent musculature from the ventral mesoderm-
segments, and the formation of the eye -muscles innervated by the

SEGMENTS OF TRIGEMINAL NERVE-GROUP 273

oculomotor nerve from the dorsal mesoderm segments. These four
segments have been so fused together that van Wijhe looked upon
tliem as a single segment, and the premandibular cavity as the cavity
of a single segment. They represent, according to my scheme, the
segments belonging to the endognaths, i.e. the second, third, fourth,
fifth pairs of prosomatic appendages ; the premandibular cavity, there-
fore, represents the second ccelomic cavity in Limulus, which, accord-
ing to Kishinouye, is the sole representative^ the coelomic cavities
of the second, third, fourth, fifth prosomatic segments. The muscles
derived from the ventral mesoderm-segments represent the muscles
of these appendages, which therefore dwindle and disappear in the
vertebrate, with the possible exception of the muscles innervated by
the descending root of the trigeminal. The muscles derived from
the dorsal mesoderm-segments, i.e. the eye-muscles supplied by the
oculomotor nerve, represent the dorso- ventral somatic muscles of these
four segments, muscles which are represented in the scorpion by the
recti group of muscles, i.e. the median dorso-preoral-entosclerite
muscles (62).

Then came two segments, the mandibular, in which muscles are
formed both from the ventral and from the dorsal mesoderm-segments.
From the former arose the main mass of muscles innervated by
the motor root of the trigeminal, from the latter the superior oblique
muscle and the mandibular muscle of Miss Piatt, of which the former
alone survives in the adult condition. These two segments are looked
upon as a single segment by van Wijhe, of which the mandibular
cavity is the coelomic cavity. They represent, according to my
scheme, the segments belonging to the sixth pair of prosomatic
appendages or ectognaths, and the seventh pair, i.e. the chilaria or
metastoma.

The first part, then, of the mandibular cavity represents the third
coelomic cavity in Limulus and the muscles derived from the ventral
mesoderm, in all probability the muscles of the tongue in the
lamprey (cf. Chap. IX.), which represents the ectognaths or sixth
pair of prosomatic appendages, while the muscles derived from the
dorsal mesoderm, i.e. the superior oblique muscles, represent the
dorso-ventral somatic muscles of this segment, muscles which are
represented in the scorpion group by the pair of anterior dorso-
plastron or oblique muscles ((33).

The second part of the mandibular cavity represents the 4th

274 THE ORIGIN OF VERTEBRATES

cceloinic cavity in Limulus aud the muscles derived from the ventral
mesoderm, in all probability the muscles of the lower lip in the
lamprey (cf. Chap. IX.), which represents the metastoma; while the
muscles derived from the dorsal mesoderm, i.e. Miss Piatt's pair of
mandibular muscles, represent the dorso-ventral somatic muscles of
this segment, muscles which are represented in the scorpion group
by the pair of median dorso-plastron muscles (64).

In connection with •this last pair of muscles we find that the
external rectus in the vertebrate represents the first dorso-ventral
mesosomatic muscle in the scorpion, i.e. the posterior dorso-plastron
muscle (65), and, as already mentioned (p. 267), that it always lies
closely alongside the mandibular muscle, just as in the scorpion group
muscle (65) always lies alongside muscle (64).

In the invertebrate as well as in the vertebrate this muscle is a
mesosomatic muscle which has taken up a prosomatic position.

The question naturally arises, what explanation can be given of
the fact that these dorso-ventral muscles attached on each side
of the mid-dorsal line to the prosomatic carapace became converted
into the muscles moving the eyeballs of the two lateral eyes ? An
explanation which must take into account not only the isolated posi-
tion of the abducens nerve, but also the extraordinary course of the
trochlearis. The natural and straightforward answer to this question
appears to me quite satisfactory, and I therefore venture to commend
it to my readers.

I have argued the case out to myself as follows : The lateral eyes
must have been originally situated externally to the group of muscles
innervated by the oculomotor nerve, for a sheet of muscle representing
the superior interned and inferior rectus muscles could only wrap
round the internal surface of each lateral eye ; i.e. the arrangement
of the muscle-sheet, as in the scorpion, about two median eyes, is in
the wrong position, for if those two eyes, which are the main eyes in
the scorpion, were to move outwards to become two lateral eyes, then
such a muscle-group would form a superior external and inferior rectus
group. The evidence, however, of Eurypterus and similar forms is
to the effect that the lateral eyes became big and the median eyes
insignificant and degenerate. If, then, with the degeneration of the
one and the increasing importance of the other, these lateral eyes
came near the middle line, then the muscular group (62), which I
have called the recti group, would naturally be pressed into their

SEGMENTS OF TRIGEMINAL NERVE-GROUP

275

service, and would form an internal and not an external group of
eye-muscles.

In Fig. 110, A, taken from Miss Beck's paper, I have shown the
relative position of the eyes and the segmental dorso-ventral pro-
somatic muscles on the carapace of the scorpion. In Fig. 110, B, I
have drawn the prosomatic carapace of EurypteruS Scouleri, taken
from Woodward's paper, with the eyes as represented there ; in this
I have inserted the segmental dorso-ventral muscles as met with in
the scorpion, thereby demonstrating how, with the degeneration of
the median eyes and the large size of the lateral eyes, the recti
muscles of the scorpion would approach the position of an internal
recti group to the lateral eyes, and so give origin to the group of
muscles innervated by the oculomotor
nerve. In the Eurypterus these large
eyes are large single eyes, not separate
ocelli, as in the scorpion.

All, then, that is recpiired is that in
the first formed fishes, which still pos-
sessed the dorso-ventral muscles of their
Eurypterid ancestors, the lateral eyes
should be the important organs of sight,
large and near the mid-dorsal line. Such,
indeed, is found to be the case. In

— f

... Occ

Fig. 111. — Dorsal Head-
Shield of Tremataspis
Mickwitzi. (From Rohon.)

amongst the masses of Eurypterids found Fr°-> narial opening; I.e., late-

,, ,-,., . , . ~. , ral eyes; flZ.,glabellum plate

in the upper Silurian deposits at Oesel, as ove/bram; 8 0cc<) occfpital

described by Eohon, numbers of the most spine.

ancient forms of fish are found belonging

to the genera Thyestes and Tremataspis. The nature of the dorsal

head-shields of these fishes is shown in Fig. 14, which represents

the dorsal head-shield of Thyestes verrucosus, and Fig. Ill that of

Tremataspis Mickwitzi. They show how the two lateral eyes were

situated close on each side of the mid-dorsal line in these Eurypterus-

like fishes, in the very position where they must have been if the

eye-muscles were derived from the dorso-ventral somatic muscles of

a Eurypterid ancestor.

In Lankester's words, one of the characteristics of the Osteostraci

(Cephalaspis, Auchenaspis, etc.), as distinguished from the Fletero-

straci (Pteraspis), are the large orbits placed near the centre of the

shield. The apparent exception of Thyestes mentioned by him is no

276 THE ORIGIN OF VERTEBRATES

exception, for orbits of the same character have since been discovered,
as is seen in Rohon's figure (Fig. 14). In Fig. 110, C, I give an
outline of the frontal part of the head-shield of a Cephalaspid. in
which I have drawn the eye- muscles as in the other two figures.

Although all the members of the Osteostraci possess large lateral
eyes towards the centre of the head-shield, the other group of ancient
fishes, the Heterostraci, are characterized by the presence of lateral
eyes far apart, situated on the margin of the head-shield on each
side (cf. Fig. 142, 0, p. 350).

So, also, on the invertebrate side, the lateral eyes of Pterygotus and
Slimonia are situated on the margin of the prosomatic carapace, while
those of Eurypterus and Stylonurus are situated much nearer the
middle line of the prosomatic carapace.

Next comes the question of the superior oblique muscle and the
trochlearis nerve. Why does this nerve (n.IV. in Fig. 106, C and D)
alone of all the nerves in the body take the peculiar position it
always does take ? The only suggestion that I know of which
sounds reasonable and worth consideration is that put forward by
Fiirbringer, which is an elaboration of the original suggestion of
Hoffmann. Hoffmann suggested in 1889 that the trochlearis nerve
represented originally a nerve for a protecting organ of the pineal
eye, which became secondarily a motor nerve for the lateral eye as
the pineal eye degenerated. Fiirbringer differs from Hoffmann in
that he considers that the nerve was originally a motor nerve, and
was not transformed from sensory to motor, yet thinks Hoffmann's
suggestion is in the right direction.

He points out that the crossing of the trochlearis is not a crossing
of fibres between two centres in the central nervous system, but may
be explained by the shifting of the peripheral organ, i.e. the muscle,
from one side to the other, and the nerve following this shift. Con-
sequently, says Fiirbringer, the course of the nerve indicates the
original position of the muscle, and therefore he imagines that the
ancestor of the superior oblique muscle was a muscle the fibres of
which were attached in the mid-dorsal line, and^interlaced with those
of the other side, the two muscles thus forming an arch through
which the nervous system with its central canal passed. Then, for the
sake of getting a more efficient pull, the crossing muscle-fibres became
more definitely attached to the opposite side of the middle line, and
finally obtained a new attachment on the opposite side, with the

SEGMENTS OF TRIGEMINAL NERVE-GROUP 2 J J

obliteration of the muscular arch ; the nerve on each side, following
the shifts of the muscle, naturally took up the position of the original
muscular arch, and so formed the trochlear nerve, with its dorsal
crossing. This explanation of Fiirbringer's was associated by him
with movements of the median pineal eyes, the length of their nerve,
according to him, even yet indicating their previous mobility. This
assumption is not, it seems to me, necessary. The length of the nerve
is certainly no indication of mobility, for in Limulus and the scorpion
group the nerve to each median eye is remarkably long, yet these
eyes are immovably fixed in the carapace. All that is required is a
pair of dorso-ventral muscles belonging to the segment immediately
following the group of segments represented by the oculomotor nerves,
the fibres of which should cross the mid-dorsal line at their attach-
ment ; for, seeing that the lateral eyes were originally so near this
position, it follows that such muscles might form part of the muscular
group belonging to the lateral eye without having previously moved
the pineal eyes. In fact, Fiirbringer's explanation requires as starting-
point that the pair of muscles which ultimately become the* superior
oblique should have the exact position of the pair of dorso-ventral
muscles in the scorpion, called by Miss Beck the anterior dorso-
plastron muscles (63), which I have named the oblique muscles.
Here, and here only, do we find an interlacement, across the mid-
dorsal line, of the fibres of attachment of the muscles on the two sides,
in consequence of which this pair of muscles is described by her as
forming an arch encircling the alimentary canal and dorsal vessel.
If, then, as I have previously argued, the primitive plastron formed a
pair of trabecular, and the nervous system grew round the alimentary
canal, such an arch would encircle the tubular central nervous system
of the vertebrate.

Still more striking is this pair of muscles (63) in Phrynus (Fig.
108), where we see how the arch formed by them almost touches
the posterior extremity of the supra-o?sophageal brain-mass, crossing,
therefore, over the beginning of the stomach region of the animal.
The angle formed by the arch is much more obtuse than that formed
in Scorpio, so that an actual crossing of the muscle-fibres has taken
place at the point of attachment to the carapace. Also, only the part
nearest the carapace is muscular, the rest forming a long tendinous
prolongation of the plastron wall (the primordial cranium), as seen in
the figure.

278

THE ORIGIN OF VERTEBRATES

This muscle-pair is, as it should be, the pair of dorso-veutral
muscles belonging to the segment immediately following on the
group of segments represented by the recti muscles, i.e. according
to previous argument, the segment belonging to the sixth pair of
locomotor appendages or ectognaths; a muscle, therefore, which
would arise in the vertebrate from the mandibular, and not from
the premandibular cavity. A similar muscle probably existed in

m nhl sup

.IV"1 nerve

CZ>

cn>

m out sup

Fig. 112. — A, Diagram of Position of Oblique Muscle in Scorpion; B, Diagram
of Transition Stage ; C, Diagram of Superior Oblique Muscle in Verte-
brate.

I.e., lateral eyes; c.e., central eyes; CIV., central nervous system; Al., alimentary

canal; c, aqueductus Sylvii.

Eurypterus (M.obl. in Fig. 106, B), and, as in the case of the for-
mation of the oculomotor group, derived from the recti group of the
scorpion, would form the commencement of the superior oblique
muscle in Thyestes and Tremataspis.

It is instructive to notice that the original position of attachment of
this muscle is naturally posterior to that of the oculomotor group of
muscles, and that Furbringer, in his description of the eye-muscles
of Petromyzon, asserts that this muscle in this primitive vertebrate

SEGMENTS OF TRIGEMINAL NERVE-GROUP 2 7 9

form is not attached as in other vertebrates, but is posterior to the
other muscles, so that he calls it the posterior rather than the superior
oblique. The nature of the change by which the muscle known in
the scorpion as the anterior dorso-plastron muscle (63) was probably
converted into the superior oblique muscle of the vertebrate, is
represented in the drawings Fig. 112, in which also are indicated
the dwindling of the median eyes, and the progressive superiority of
the lateral eyes, as well as the transformation of the recti muscle-
group of the scorpion into the muscles supplied by the oculomotor
nerve of the vertebrate.

With respect to the external rectus muscle, it follows naturally
that if the muscles (64) and (65) are to follow suit with the rest of
the group and become attached to the lateral eyes, they must take
up an external position. These two muscles, which always run
together, as seen in Fig. 110, A, the one belonging to the prosoma
and the other to the mesosoma, are represented by the mandibular
muscle of Miss Piatt and the external rectus, the former derived
from the walls of the last pro-otic head-cavity, the latter from the
foremost of the opisthotic head-cavities.

Such, then, is the simple explanation of the origin of the eye-
muscles which follows from my theory, and we see that the successive
alterations of the position of the orbit, and, therefore, of the globe of
the eye with its muscles, as we pass from Thyestes to man, is the
natural consequence of the growth of the frontal bone, i.e. of the brain.

The Trigeminal Nerves and the Muscles supplied by them.

Turning now to the evidence as to the number of ventral seg-
ments, i.e. the motor and sensory supply to the prosomatic appendages
afforded by the trigeminal nerve, we must, I think, come to the same
conclusion as Dohrn, viz. that if there were originally seven dorsal or
somatic segments in this region represented by : 1, Anterior cavity,
muscle lost ; 2, 3, 4, 5, muscles of the premandibular cavity, swp. rectus,
inf. rectus, int. rectus, inf. oblique, supplied by Illrd nerve; 6, 7,
muscles of the mandibular cavity, sup. oblique, supplied by IVth nerve
and muscle lost, there must have been also seven corresponding
ventral or splanchnic segments supplied by the trigeminal. At present
the evidence for such segments is nothing like so strong as for the
corresponding somatic ones; there are, however, certain suggestive

280 THE ORIGIN OF VERTEBRATES

facts which point distinctly in this direction in connection with both
the motor and sensory parts of the trigeminal. The origin of the
trigeminal motor fibres in the central nervous system is most striking.
We may take it for granted that a nucleus of cells giving origin to
one or more segmental motor nerves will possess a greater or less
longitudinal extension in the central nervous system, according to
the number of fused separate segmental centres it represents. Thus
a nucleus such as that of the IVth nerve or of the facial is small
and compact in comparison to the extensive conjoint nucleus of
the vagus and cranial accessory.

Upon examination of the motor nucleus of the trigeminal, we
find a compact or well-defined nucleus, the nucl. masticator ius, the
nerves of which supply the masseter, temporal, and other muscles,
so that the anatomical evidence at first sight appears to bear out
van Wijhe's conclusion that the motor trigeminal supplies at most
two segments. Further examination, however, shows that this is not
all, for the extraordinary so-called descending root of the Vth must
be taken into consideration in any question of the origin of the
motor elements, just as the equally striking ascending root enters
into the consideration of the meaning of the sensory elements of
the Vth.

It is not necessary here to discuss the controversy as to whether
this descending root is motor or sensory. It is universally con-
sidered at present to be motor, and is believed to supply, as
Kolliker suggested, among other muscles, the m. tensor tympani and
the m. tensor veli palati. It is thus described by Obersteiner —

" From the region of the mid-brain the motor root receives an
important addition of thick fibres, which form the cerebral or
descending root. The large, round vesicular cells from which the
fibres of the descending root arise form no single compact group, but
are partly single, partly arranged like little bunches of grapes, as far
as the region of the anterior corpora quadrigernina. The further we
go brainwards, the smaller is the number of fibres. In the region
of the anterior corpora quadrigernina, the few cells of origin are
found more and more median ; so that the uppermost trigeminal
fibres descend in curves almost from the mid-line, as is shown by the
exceptional occurrence of one or more of the characteristic cells above
the aqueduct. At the height of the posterior commissure one finds
the last of these trigeminal cells."

SEGMENTS OF TRIGEMINAL NERVE-GROUP 28 1

The anatomy of the Vth nerve reveals, then, three most striking
facts : —

1 . The motor nucleus of the Vth extends from the very commence-
ment of the infra-infundibular region to nearly the commencement
of the nucleus of the Vllth ; in other words, the motor nucleus of the
Vth extends through the whole prosomatic region, just as it must
have done originally if its motor nerves supplied the muscles of
the prosomatic appendages. Such an extended range of origin is
indicative of the remains of an equally extended series of segmental
centres or ganglia.

2. Of these centres the caudalmost have alone remained lame and
vigorous, constituting the nucleus masticator ins, which in the fish is
divided into an anterior and posterior group, thus indicating a
double rather than a single nucleus ; while the foremost ones have
dwindled away until they are represented only by the cells of the
descending root, the muscles of these segments being still represented
by possibly the tensor veli palati and the other muscles innervated
from these cells.

3. The headmost of these cells takes up actually a position dorso-
lateral to the central canal, so that the groups on each side nearly
come together in the mid-dorsal line ; a very unique and extra-
ordinary position for a motor cell-group, but not improbable when we
recall to mind Brauer's assertion as to the shifting of the foremost
prosomatic ganglion-cells of the scorpion from the ventral to the
dorsal side of the alimentary canal.

On the sensory side the evidence is also suggestive, the question
here being not so much the distribution of the sensory nerves as the
number of ganglia belonging to each of the cranial nerves.

With respect to this question, morphologists have come to the
conclusion that there is a marked difference between spinal and
cranial nerves, in that whereas the posterior root- ganglia of the
spinal nerves arise from the central nervous system itself, i.e. from
the neural crest, the ganglia of the cranial nerves arise partly from
the neural crest, partly from the proliferation of cells on the surface
of the animal ; and because of the situation of these proliferating
epidermal patches over the gill-clefts in the case of the vagus and
glossopharyngeal nerves, they have been called by Froriep and Beard
branchial sense-organs. Beard divides the cranial ganglia into two
sets, one connected with the neural ridges, called the neural ganglia,

282 THE ORIGIN OF VERTEBRATES

and the other connected with the surface-cells, which he calls the
lateral ganglia. This second set corresponds to Kupffer's epibranchial
ganglia. Now it is clear that in the case of the vagus nerve, where,
as is well shown in Ammoccetes, the nerve is not a single segmental
nerve, but is in reality made up of a number of nerves going to
separate branchial segments, the indication of such segments is not
given by the main vagus ganglion or neural ganglion, but by the series
of lateral ganglia. So also it is argued in the case of the trigeminal,
that if in addition to the ganglion-cells arising from the neural crest
separate ganglion-masses are found in the course of development,
in connection with proliferating patches of the surface (plakodes,
Kupffer calls them), then such isolated lateral ganglia are indications
of separate segments, just as in the case of the vagus, even though
the separate segments do not show themselves in the adult. So far
the argument appears to me just, but the further conclusion that the
presence of such plakodes shows the previous existence of branchial
sense-organs, and, therefore, that such ganglia are epibranchial
ganglia, indicating the position of a lost gill-slit, is not justified by
the premises. If, as I suppose, the trigeminal nerve supplied a series
of non-branchial appendages serially homologous with the branchial
appendages supplied by the vagus, then it is highly probable that the
trigeminal should behave with respect to its sensory ganglia similarly
to the vagus nerve, without having anything to do with branchiae.

Such plakodal ganglia, then, may give valuable indication of 11011 -
branchial segments as well as of branchial segments. The researches
of Kupffer on the formation of the trigeminal ganglia in Ammoccetes
are the chief attempt to find out from the side of the sensory ganglia
the number of segments originally belonging to the trigeminal. The
nature and result of these researches is described in my previous
paper (Journal of Anatomy and Physiology, vol. xxxiv.), and it will
suffice here to state that he himself concludes that the trigeminal
originally supplied five at least, probably six, segments. As I have
stated there, the evidence as given by him seems to me to indicate
even as many as seven segments.

In the full-grown Ammoccetes, as is well known, there are two
distinct ganglia belonging to the trigeminal, the one the gauglion of
the ramus ophthalmicus, the other the main ganglion.

According to Kupffer the larval Ammoccetes possesses three sets
of ganglia, not two, for between the foremost and hindmost ganglion

SEGMENTS OF TRIGEMINAL NERVE-GROUP

he describes a nerve (x., Fig. 113), with four epibranchial ganglia,
which do not persist as separate ganglia, but either disappear or are
absorbed into the two main ganglia (Fig. 113). This discovery of
Kupffer's is very suggestive, for, as already stated, a transformation
takes place when the Ammoccetes is 5 mm. long, so that the
arrangement of the parts before that period is distinctly more
indicative of the ancestral arrangement than any later one.

If we use the name plakodal ganglia to represent that part of
these ganglia which was originally connected with the skin, then

Vxvn ix x

Fig. 113. — Ganglia op the Cranial Nerves of an Ammoccetes, 4 mm. in length,

PROJECTED ON TO THE MEDIAN PLANE. (After KUPFPER.)

A-B, the line of epibranchial ganglia; an., auditory capsule; nc., notochord ; Hy.,
tube of hypophysis ; Or., oral cavity; u.l., upper lip ; l.l. lower lip; vel., septum
between oral and respiratory cavities ; V., VII., IX., X., cranial nerves ; x.,
nerve with four epibranchial ganglia.

Kupffer's researches assert that in the larval Ammoccetes there were
seven such plakodal ganglia, one in front belonging to the foremost
trigeminal ganglion, two behind, parts of the hindmost ganglion, and
four in between, which do not exist later as separate ganglia.

In accordance with the views put forward in this book, a possible
interpretation of these plakodal ganglia would be given as follows : —

Beard, who, after Froriep, drew attention to this relation of the
cranial ganglia to special skin-patches, has compared them with the
parapodial ganglia of annelids, i.e. ganglia in connection with
annelidan appendages ; whether we are here obtaining a glimpse of
the far-off annelidan ancestry of both arthropods and vertebrates it
would be premature at present to say. It is natural enough to
expect, on my view, to find evidence of annelidan ancestry in

284 THE ORIGIN OF VERTEBRATES

vertebrate embryology (as has been so often asserted to be the case),
seeing that undoubtedly the Arthropoda are an advanced stage of
Annelida ; and, indeed, the way is not a long one when we consider
Beecher's evidence that the Trilobita belong to the Phyllopoda,
certainly a primitive crustacean group, which Bernard derives directly
from the annelid group Chretopoda. If, then, these plakodal ganglia
indicate the former presence of appendages, we obtain this result : —
The foremost ganglion on each side possesses one plakodal ganglion,
and therefore indicates an anterior pair of appendages, possibly the
chelicerse. Then comes the peculiar nerve with four plakodal
ganglia indicating on each side four appendages close together,
possibly the endognaths. Then, finally, on each side, the second
large ganglion with two plakodal ganglia, indicating two pairs of
appendages, possibly the ectognaths and the metastoma.

Summary.

The consideration of the history of the cranial segmentation shows that
whereas, from the commencement of that history, the evidence for two ventral
segments supplied by the trigeminal nerve is clear and unmistakable, later
observers have tended more and more to increase the number of these segments,
until at the present time the evidence is in favour of at least six, probably seven,
as the number of segments supplied by the motor part of the trigeminal.

So. also, the original evidence for the number of dorsal or somatic segments
limits the number to three, innervated respectively by the oculomotor (III.),
trochlear (IV.), and abducens (VI.) nerves, or rather two, since the last nerve
belongs to the facial segment. The muscles which these three nerves supply
are derived respectively from the walls of the premandibular. mandibular, and
hyoid coelomic cavities.

Later evidence points strongly to the conclusion that the oculomotor nerve
and the premandibular cavity represent not one segment but the fusion of
four, while the mandibular cavity represents two segments. In addition to
these. Miss Piatt has discovered a still more anterior head-cavity, which she has
named the anterior cavity, so that the pro-otic segments on this reckoning are
seven in number, viz. : (1) the anterior cavity, (2, 3, 4, 5) the premandibular cavity,
(6, 7) the mandibular cavity. The somatic muscles belonging to these dorsal
segments are the eye-muscles, which are all dorso-ventral in position, and are
not the same as the longitudinal somatic muscles, but belong to a distinct dorso-
ventral segmental group, the only representative of which at present known in
the mesosomatic region is the external rectus innervated by the Vlth nerve.

These head-cavities, and these muscles of the vertebrate, resemble the
corresponding cavities and muscles of the invertebrate to an extraordinary

SEGMENTS OF TRIGEMINAL NERVE-GROUP 285

degree, so that it becomes easy to see how the dorso-ventral muscles of the
prosomatic segments of the latter have become converted into the eye-niuscu-
lature of the former. The most powerful proof of all that such a conversion
has taken place is that a natural and simple explanation is at once given of
the extraordinary course taken by the IVth or trochlear nerve. Ever since
neurology began, the course of this nerve has arrested the attention of anato-
mists. Why should just this one pair of nerve-roots of all those in the whole
body be directed dorsalwards instead of ventralwards, and cross each other in the
valve of Vieussens, each to supply a simple eye-muscle (the superior oblique)
belong-ing to the other side ? For generations anatomists have wondered and
found no solution, and yet, without any straining of hypotheses, in consequence
simply of the investigation of the anatomy of the corresponding pair of muscles
in the scorpion group, the solution is immediately apparent.

This pair of muscles alone, of all the musculature attached to the carapace,
crosses the mid- dorsal line to be attached to the other side, thus carrying its
nerve with it to the other side ; by a continuation of the same process the
relation of the trochlear to the superior oblique muscle can be explained.

The comparison of the eye-muscles of the vertebrate with the dorso-ventral
segmented muscles of the invertebrate makes the number and nature of the
pro-otic segments much clearer.

CHAPTER IX

THE PROSOMATIC SEGMENTS OF AMMOCCETES

The prosomatic region in Ammocoetes. — The suctorial apparatus of the adult
Petroniyzon. — Its origin in Ammocoetes. — Its derivation from appendages.
— The segment of the lower lip or metastomal segment. — The tentacular
segments. — The tubular muscles. — Their segmental arrangement. — Their
peculiar innervation. — Their correspondence with the system of veno-
pericardial muscles in Limulus. — The old mouth or palreostoma. — The
pituitary gland. — Its comparison with the coxal gland of Limulus. —
Summary.

In the last chapter it was seen not to be incompatible with both the
anatomical and morphological evidence to look upon the trigeminal
nerves as having originally supplied the seven prosomatic pairs of
appendages of the invertebrate ancestor, the foremost of which, the
chelicerse, and the four pairs of endognaths dwindled away and became
insignificant, leaving as trace of their former presence the descending
root of the Vth nerve ; while the two hindmost pairs, the ectognaths
and the chilaria, or metastoma, remained vigorous and developed,
leaving as proof of their presence the nucleus masticatorius. Evi-
dence in favour of this suggestion and of the nature of the dwindling
process is afforded when we examine what the trigeminus does supply
in Ammocoetes. In all vertebrates this nerve supplies the great
muscles of mastication which, in all gnathostomatous fishes, move
the jaws. The lowest fishes, the cyclostomes, possess no jaws ; they
take in their food by attaching themselves to their prey and by
means of rasping teeth situated in serried rows within the circular
mouth, combined with a powerful suctorial apparatus, they suck the
juices of the fish they feed upon. Not possessing jaws, they feed
by suction on the living animal, a method of feeding which gives
them no more claim to be classed as parasitic animals than the
whole group of spiders which feed in a similar manner on living
flies.

THE PROS OM A TIC SEGMENTS OF AMMOCCETES 287

The Origin of the Suctorial Apparatus of Petromyzon.

This powerful suctorial apparatus is innervated entirely by the
trigeminal nerve, so that here in its muscular arrangements any
original segmental arrangement of the muscles of mastication might
be expected to be visible, It consists of a large rod or piston, to
which are attached powerful longitudinal muscles ; a large muscle,
the basilar muscle, which assists the piston in producing a vacuum,
and annular muscles around the circular lip.

Turn now to the full-grown larval form, Ammoccetes, an animal
in the case of Petromyzon Plancri as large as the full-grown Petro-
myzon, and seek for this musculature. There is, apparently, no sign
of it, no suctorial apparatus whatever, only, as already mentioned,
an oral chamber bounded by the lower and upper lips and the
remains of the septum between it and the respiratory chamber — the
velar folds. Attached to its walls a number of tentacles are situated,
which form a fringe around and within the mouth. Most extra-
ordinary is the contrast here between the larval and the adult
stages ; in the former, no sign of the suctorial apparatus, but simply
tentacles and velar folds ; in the latter, no sign of tentacles or of
velar folds, but a massive suctorial apparatus.

In order, then, to understand the origin of the muscles of masti-
cation, it is necessary to study the changes which occur at trans-
formation, and thus to find out how the suctorial apparatus of the
adult arises. This most important investigation has been under-
taken by Miss Alcock, and owing to the kindness of Mr. Millington,
of Thetford, we have been able to obtain a better series in the trans-
formation process than has ever been obtained before. Miss Alcock
has not yet published her researches, but has allowed me to make
use of some of her facts.

An enormous proliferation of muscular tissue takes place with
great rapidity during this transformation, which causes the disappear-
ance of the tentacles, and gives origin to the suctorial apparatus.
The starting point of this proliferation can be traced back in all
cases to little groups of embryonic tissue found below the epithelial
lining of the oral chamber in Ammoccetes. Of these groups the most
conspicuous one is situated at the base of the large median ventral
tentacles. Others are situated at the base of the tentacular ridge.
Further, although this extraordinary change takes place in the

:88

THE ORIGIN OF VERTEBRATES

peripheral organ, no marked difference occurs in the arrangement
of the nerves issuing from the trigeminal motor centre, no new
nerves are formed to supply the new muscles, but every motor nerve-
fibre and the motor cell from which it arises increases enormously in
size, and these giant nerve-fibres thus formed split into innumerable
filaments corresponding with the proliferation of the muscular elements.
The clue, then, to the origin of the suctorial apparatus and of the
nature of the original organs supplied by the trigeminal is afforded
in this case, as in all other similar inquiries, by the central nervous
system and its outgoing nerves. Here is always the citadel, the
fixed seat of government, here is ' headquarters,' from which the
answers to all our inquiries must originate.

The Trigeminal Nerve of Ammoccetes.

Striking is the answer. In Fig. 114, Miss Alcock has drawn the
distribution of the trigeminal nerve as traced by her through a series

Diet. pslbf.

Fig. 114. — Distribution op Trigeminal Nerve in Ammoccetes.

ps. br., pseudo-branchial groove; met., nerve to lower lip, or metastomal nerve; /.,
nerve to tongue ; tent., nerve to tentacles. The mandibular and internal maxil-
lary nerves are coloured red ; the purely sensory nerves to the external surface
are coloured black.

of sections. It arises, as is well known, from two separate ganglia, of
which the foremost gives rise to a purely cutaneous nerve, the oph-
thalmic nerve, and the hindmost to three nerves, the most posterior
of which is purely cutaneous and passes tailwards over the ventral
branchial region, as shown in the figure ; the other two nerves, both

THE PROS OM 'A TIC SEGMENTS OF AMMOCCETES 289

of which contain motor fibres, are called by Hatschek the mandibular
and maxillary nerves. Of these the mandibular or velar nerve {met)
is a large, conspicuous nerve, which arises so separately from the
rest of the trigeminal as almost to deserve the title of a separate
nerve. When it leaves the large posterior ganglion, it passes into
the anterior part of the velum, runs along with the tubular muscles,
which it supplies, to the ventral surface as far as the junction of the
lower lip with the thyroid plate, and has not been followed further by
Hatschek. Miss Alcock, however, by means of serial sections, has
traced it further, and shown that at this point it turns abruptly
headwards to terminate in the muscles of the lower lip. If, then,
as suggested, the lower lip represents the metastoma — the last pair
of prosomatic appendages— then this mandibular or velar nerve
represents that segmental nerve.

The other nerve — the maxillary nerve of Hatschek — which con-
stitutes the larger part of the trigeminal, passes forwards from the
ganglion, and at a point somewhere about the anterior region of the
eyeball, divides into two, an external (black in Fig. 114) and an
internal (red in Fig. 114) nerve. The external branch is apparently
entirely sensory, and supplies the external surfaces of the upper and
lower lips. The internal branch is mainly motor, and supplies the
muscles of the upper lip ; it contains also the nerves of the tentacles.

The nerve to the median ventral tentacle (t.) or tongue leaves the
internal division of the maxillary immediately after its separation
from the external ; it runs ventralwards, and at the same time passes
internally until it reaches a position between the muco-cartilage and
the epithelium lining the cavity of the throat. It then turns, and
passing posteriorly (towards the tail) to the point where the median
ventral tentacle is attached to the lower lip, it supplies some very
rudimentary-looking muscles which run from the tentacle to the
adjoining surface, and no doubt serve to move the tentacle from side
to side. A portion of the nerve still continues to run along the side
of the median ventral ridge, as far back as the point where the
muscles of the hyoid segment pass round to the ventral side between
the velum and the thyroid ; in fact, this small nerve passes along
the whole length of the median ventral ridge.

This description shows that the trigeminal nerve divides itself
into two groups : the one represented black in the figure, which is
purely cutaneous and sensory, corresponding, in the main, according

290 THE ORIGIN OF VERTEBRATES

to my theory, to the epimeral nerves of Limulus ; the other coloured
red, which supplies muscles belonging to the visceral or splanchnic
muscle-group, and contains also the nerves to the tentacles.

This latter group, which is formed by two distinct well-defined
nerves, viz, the mandibular and the internal branch of the maxillary,
corresponds, according to my theory, to the amalgamated nerves of
the prosomatic appendages, and is clearly divisible into three distinct
nerves —

1. The lower lip-nerve or the metastomal nerve {met.).

2. The tongue-nerve (t.).

3. The nerve (tent.) to the upper lip and tentacles.

Of these three pairs of nerves it is suggested that the first pair
were derived from the nerves to the metastomal appendage. The
second pair of nerves ought, on this theory, originally to have sup-
plied the pair of appendages immediately in front of the metastoma
— that is, the pair of ectognaths, and therefore the ventral pair of
tentacles, known as the tongue, would represent the last remnant of
these ectognaths. Similarly, the other tentacles would represent the
enclognaths, and therefore the third pair of nerves would represent
the fused nerves to these concentrated endognaths, which, in the
Eurypterids, stand aloof from the ectognaths.

Let us consider these three propositions separately. In the first
place, have we any right to attribute segmental value to the man-
dibular nerve ? What evidence is there of segments in this region
in Ammocoetes ?

The Segment of the Lower Lip, or Metastomal Segment.

We have seen that in the branchial or mesosomatic region the
segments corresponding to the mesosomatic appendages were mapped
out by means of their supporting or skeletal structures, their seg-
mental muscles, and their nervous arrangements, as well as by the
arrangement of the branchiae. Similarly, the segments in front of
the branchial region, corresponding to the prosomatic appendages,
ought to be definable by the same means, although, owing to the
absence of branchiae and the greater concentration in this region,
the separate segments would probably not be so conspicuous.

The last segment considered was the segment belonging to the
Tilth nerve corresponding to the opercular appendages of the

THE PROSOMATIC SEGMENTS OF AMMOCCETES 29 1

Eurypterid. The segment immediately in front of this is the next
for consideration, viz. that corresponding to the chilarial appendages
or metastoma ; and as the basal part of this pair of appendages was
fused with the basal part of the operculum, the one cannot be dis-
cussed without the other ; therefore, the segment to which the lower
lip belongs must be considered in connection with and not apart
from the thyro-hyoid segments already dealt with.

In Chapter V., p. 188, I stated that the supporting bars of the
foremost mesosomatic segments, the thyro-hyoid segments, differed
from the cartilaginous bars of the branchial segments, in that they
were composed of muco-cartilage. Also in addition to the muco-
cartilaginous skeletal bars, a ventral plate of muco-cartilage exists in
Ammoccetes which covers over the thyroid gland.

Similarly in the prosomatic segments the skeletal bars are com-
posed of muco-cartilage and the ventral plate of muco-cartilage
continues forward as the plate of the lower lip. It is of special
interest, in connection with the segments indicated by such support-
ing structures, to find that this special tissue is entirely confined to
the head-region, and disappears absolutely at transformation, thus indi-
cating the ancestral nature of the segments marked out by its presence.

This muco-cartilaginous skeleton is the key to the whole position,
and requires, therefore, to be understood. It is of great importance,
not only because it demonstrates the position of the segments in
Ammoccetes which characterized its invertebrate ancestor, but also
because it possesses a structure remarkably similar to that found
in the head-plates of the most ancient fishes. For the present I will
confine myself to the consideration of this muco-cartilaginous skeleton
as evidence of the relationship of Ammoccetes to the Eurypterids,
and in the next chapter will show how absolutely the same skeleton
corresponds to that of the Cephalaspidre, so that Ammoccetes is
really a slightly modified Cephalaspid, the larval form of which was
Eurypterid in character.

In Chapter IV., Eigs. 63, 64, I have given a representation of the
ventral and dorsal views of an Ammoccetes cut in half horizontally.
Such a section shows with great clearness the series of branchial
appendages with their segmental muscles and cartilaginous bars
which form the branchial segments innervated by the IXth and Xth
nerves, according to my view of the branchial unit. As is seen (Fig.
64 or 115), the skeletal bar of the hyoid or opercular appendage,

Tr. -^

Ser.--

Fig. 115.— Dorsal
half of Head-
region of Am-
moccetes.

Inf.

Tr., trabecule ;
Pit., pituitary

space ; Inf., in-
f u ri d i b n 1 u m ;
Ser.. median ser-
rated flange of
velar folds.

THE PROSOMATIC SEGMENTS OF AMMOCCETES 293

which is clearly serially homologous with the other branchial bars, is
composed of muco-cartilage, and not of cartilage. If we follow this
series of horizontal sections nearer to the origin of the cartilaginous

Fig. 116. — Horizontal Section through the Anterior Part of Ammocoetes,

IMMEDIATELY YeNTRALLY TO THE AUDITORY CAPSULE.

s/.-,-sA-5) skeletal bars; my-ms, striated visceral muscles; mt^-mt^ tubular muscles;
b)\-br3, brancbiee ; ir., trabecule; inf., iufuudibulum ; ped., pedicle; V., tri-
geminal nerve. Muco-cartilage, red ; soft cartilage, blue ; bard cartilage, purple.

bars from the sub-chordal cartilaginous rod on each side of the noto-
chord, we obtain a picture, as in Fig. 116, in which each branchial
segment is defined by the section of the branchial cartilaginous bar

294

THE ORIGIN OF VERTEBRATES

(,s7t4, shs), by tlie section of the separate branchiae (&r2, br3), and by the
separate segmental muscles arranged round each bar, these muscles
being partly ordinary striated (m4, w5), partly tubular (mts, rati). The
uppermost of these branchial segments shows the same arrangement ;
(.s7,'3) is the branchial skeletal bar, which is now composed of niuco-
cartilage, not cartilage ; (h\) is the branchiae in the same situation as
the others, but here composed of glandular rather than of respiratory
epithelium, while the ordinary striated branchial muscles of this seg-
ment are marked as (?%), being separated from the tubular muscles of
the segment (m£2)3 owing to the large size of the blood-space in which

aud

mL eye

mti

Fig. 117. — Sagittal Lateral Section through the Anterior Part op Ammocctites.
Lettering and colouring same as in Fig. 116. and., auditory capsule ; j.v., jugular vein.

these latter muscles are lying. In front of this segment so defined
we see again another well-marked skeletal bar (s7c2) of muco- cartilage,
evidently indicating a similar segment anterior to the hyoid segment.
In connection with this bar there are no branchiee, but a^ain we see
two sets of visceral muscles, the one ordinary striated, marked (m2),
and the other tubular, marked (mti). Here, then, the section indicates
the existence of a segment of the same character as the posteriorly
situated branchial segments but belonging to a non-branchial region
— a segment which would represent a non-branchial appendage, the
last, therefore, of the prosomatic appendages. Let us, then, follow

THE PROSOMATIC SEGMENTS OF AMMOCCETES 295

out these two segmental muco-cartilaginous bars and their attendant
muscles, and see to what sort of segments their investigation
leads.

The bar which comes first for consideration (sl3) arises imme-
diately behind the auditory capsule from the first branchial cartilage
very soon after it leaves the sub-chordal cartilaginous ligament ; the
soft cartilage of the sub-chordal ligament ceases abruptly in its
extension along the notochord at the place where the hard cartilage
of the parachordal joins it, and in a sense it may be said to leave the
notochord at this place and pass into the basal part of the first branchial
bar. The most anterior continuation of this branchial system is this
muco-cartilaginous bar (sk3), which passes forward and ventral wards,
being separated from the axial line by the auditory capsule (cf. Fig.
118, A, B, C). Its position is well seen in a sagittal section, such
as Fig. 117. It follows absolutely the line of the pseudo-branchial
groove (ps. br., Fig. Ill), and ventrally joins the plate of muco-
cartilage which covers the thyroid gland. It forms a thickened
border to this plate anteriorly, just as the branchial cartilaginous
bars border it posteriorly. In fact, it behaves with respect to the
hyoid segment in a manner similar to the rest of the cartilaginous
bars with respect to their respective segments.

It represents, although composed of muco-cartilage, the cartila-
ginous bar of the operculum in Limulus, which also forms the termi-
nation of the branchial cartilaginous system, as fully explained in
Chapter III. ; it may therefore be called the opercular bar.

The next bar (s&2) is extremely interesting, as we are now out of
the branchial or mesosomatic region, and into the region corresponding
to the prosoma. It starts from a cartilaginous projection made of
hard cartilage, just in front of the auditory capsule, called by Parker
the 'pedicle of the pterygoid' — a projection (ped.) which defines the
posterior limit of the trabecular on each side, where they join on to
the parachordals, — and winding round and below the auditory capsule,
joins the opercular bar (cf. Fig. 118), to pass thence into and form part
of the muco-cartilaginous plate of the lower lip. In the section figured
(Fig. 116), this projection of hard cartilage is not directly continuous
with (sk^), owing to a slight curvature in the bar ; the next few
sections show clearly the connection between (ped.) and (sh-z), and
consequently the complete separation by means of this bar of the
hyoid segment from the segment in front. In the figures, the hard

296

THE ORIGIN OF VERTEBRATES

ski

Fig. 118.— Skeleton of Head-Region of Ammococtes. A, Lateral View; B,

Ventral View ; C, Dorsal View.

Muco-cartilagc, red ; soft cartilage, blue ; hard cartilage, pttrjilc. sku sk„, sk3,
skeletal bars ; ex., position of pineal eye ; na. cart., nasal cartilage ; peel., pedicle ;
o\, cranium ; nc., notochord.

THE PROSOMATIC SEGMENTS OF AMMOCCETES 297

cartilage is coloured purple, the soft cartilage blue, and the rnuco-
cartilage red, so that the position of this bar is well shown. This
bar may be looked upon as bearing the same relation to the niuco-
cartilaginous plate of the lower lip as the opercular bar does to the
muco-cartilaginous plate over the thyroid ; and seeing that these two
plates form one continuous ventral head-shield of muco-cartilage
(Fig. 118, B), and also that this bar fuses with the opercular bar, we
may conclude that the segment represented by the lower lip is
closely connected with the hyoid or opercular segments. In other
words, if the lower lip arose from the metastoma, then this pair of
skeletal bars might be called the metastomal bars, which formed the
supporting skeleton of the last pair of prosomatic appendages and, as
is likely enough, arose in connection with the posterior lateral horns
of the plastron; these posterior lateral horns, like the rest of the
plastron, would give rise to hard cartilage, and so form in Ammoccetes
the two lateral so-called pterygoid projections.

In the branchial region the muscles which marked out each
branchial segment wrere of two kinds — ordinary striated visceral
muscles and tubular muscles. Of these the former represented the
dorso-ventral muscles of the branchial appendages, while the latter
formed a separate group of dorso-ventral muscles with a separate
innervation which may have been originally the segmental veno-
pericardial muscles so characteristic of Limulus and the scorpions.
In Figs. 116, 117, the grouping of these muscles in each branchial
segment is well shown, and it is immediately seen that the hyoid
segment possesses its group of striated visceral muscles (ra3) supplied
by the Vllth nerve in the same manner as the posterior groups, as
has already been pointed out by Miss Alcock in her previous
paper. Passing to the segment in front, Fig. 116 shows that the
group of visceral muscles (m2) corresponds in relative position with
respect to the metastomal bar to the hyoid muscles with respect
to the opercular bar or to the branchial visceral muscles with
respect to each branchial bar. What, then, is this muscular group ?
The series of sections show that these are the dorso-ventral muscles
belonging to the lower lip, which, as seen in Fig. 119 (M.), form a
well-marked muscular sheet, whose fibres interlace across the mid-
ventral line of the lower lip. This group of lower lip-muscles is very
suggestive, for these muscles arise, not from the trabecula3, but from
the front dorsal region of the cranium, just in front of the two lateral

298

THE ORIGIN OF VERTEBRATES

eyes. In Fig. 117 the dorsal part is seen cut across on its way to its
dorsal attachment. Such an origin is reminiscent of the tergo-coxal
group of muscles, arising, as they do, from the primordial cranium
and the tergal carapace, and suggests at once that when the chilarial

appendages expanded to form a meta-
stoma, their tergo-coxal muscles formed
a sheet of muscles similar to those of
the lower lip of Ammoccetes, by which
the movements of the metastoma were
effected. The posterior limit of these
muscles ventrally marks out the junction
of the segment of the lower lip with
that of the thyroid ; in other words,
indicates where the metastoma had fused
ventrally with the operculum (Fig. 117).
Besides the striated visceral muscles,
each branchial segment possesses its own
tubular muscles, shown in Fig. 116 (mt3)
and (mti). As the section shows, there
is clearly a group of tubular muscle-
fibres belonging to the hyoid segment
(////•2), and also another group belonging
to the segment in front of the hyoid
(mti); so that, judging from this section,
each of these segments possesses its
own tubular musculature just as do the
branchial segments, the difference being
that the tubular muscles are more
separated from the striated visceral
group than in the true branchial seg-
ments, owing to the size of the blood-
spaces surrounding them. What, then,
are these two groups of muscles ?
Tracing them in the series of sections,
both groups are seen to belong to the system of velar muscles,
forming an anterior and a posterior group respectively ; and we see,
further, that there is not the slightest trace of any tubular muscles
anterior to these muscles of the velum.

In the living Ammoccetes the velar folds on each side can be seen

Fig. 119.— Ventual View op
Head-Region of Ammoccetes.

Th., thyroid gland; M., lower
lip, with its muscles.

THE PROSOMATIC SEGMENTS OF AMMOCCETES 299

to move .synchronously with the movements of respiration, con-
tracting at each expiration, and thus closing the slit by which the
oral and respiratory chambers communicate, and so forcing the
waters of respiration through the gill-slits, as described by Schneider.
•Such a fact is clear evidence that these tubular muscles of the velar
folds belong to the same series as the tubular muscles of the branchial
segments, so that if, as I have already suggested, the latter muscles
were originally the veno-pericardial muscles of segments corre-
sponding to the branchial appendages, then the former would represent
the veno-pericardial muscles of the segments corresponding to the
opercular and metastomal appendages. What, then, are these velar
folds, and how is it that the tubular muscles of these two segments
become the velar muscles ? I will consider, in the first instance, the
posterior group of muscles (mt2) in Fig. 116.

It has already been pointed out that the tubular muscles of the
branchial segments are dorso-ventral, but do not run with the
ordinary constrictors, having separate attachments and running part
of their course internally to and part externally to the ordinary con-
strictors. At first sight, as is usually stated, the hyoid segment does
not appear to possess tubular muscles at all. If, however, we follow
the posterior group of velar muscles (mt.2), we see (Fig. 117) that
they pass between the auditory capsule and the opercular bar (sk3) of
muco-cartilage to reach the region of the jugular vein (/. v.) posteriorly
to the auditory capsule, so that their dorsal origin bears the same
relation to the hyoid segment as the dorsal attachment of the rest of
the tubular muscles to their respective segments. Further, these
muscles run along the length of the velar fold, and are attached
ventrally on each side of the thyroid gland, so that their ventral
attachment also corresponds in position, as regards the hyoid segment,
with the ventral attachment of the rest of the tubular muscles as regards
their respective segments.

This ventral attachment is shown in Fig. 119 on each side of the
thyroid, and in Fig. 120 {mt2) ; while in Fig. 117 the fibres are seen
converging to this ventral position. In other words, this large
posterior muscle of the velar folds is a dorso-ventral muscle, and
would actually take the same position in the hyoid segment as the
dorso-ventral tubular muscles in the other branchial segments, if
the velum were put back into its original position as the septum ter-
minating the branchial chamber. Conversely, the presence of these

300 THE ORIGIN OF VERTEBRATES

hyoid tubular muscles in the velum gives evidence that the oper-
cular segment takes part in the formation of the septum, as already
suggested.

Miss Alcock, in her paper, speaks of tubular muscles belonging
to the hyoid segment, which are attached to the muco-cartilage. *
Schaffer also speaks of certain tubular muscles belonging to the velar
group as piercing the muco-cartilage (h. r. s.) in his figures 24 and 25,
i.e. the metastomal bar, near its junction with the opercular bar. In
my specimens there is a distinct group of tubular muscles which
pierce the opercular bar of muco-cartilage at its junction with the
metastomal bar, and pass into the posterior group of velar muscles.
They clearly belong to the hyoid segment, as Miss Alcock supposed,
but are not attached to the muco-cartilage. It is possible that they
represent a different group to those already considered, and suggest
the possibility that this opercular or thyro-hyoicl segment is double
with respect to its original veno -pericardial muscles as well as in
other respects.

The anterior group of tubular muscles (mth Figs. 116, 117)
belonging to the same segment as the metastomal bar must now be
taken into consideration. Very different is their origin to that of
the posterior group : they arise close up against the eye, and have
given rise to Kupffer's and Hatschek's misconception that the superior
oblicpie muscle of the eye arises from a part of the velar muscu-
lature. Naturally, as Neal has pointed out, they have nothing to do
with the eye-muscles ; the superior oblique muscle is plainly in its
true place entirely apart from these velar muscles, which form the
foremost group of the segmental tubular muscles. They pass into
the anterior part of the velar folds and run round to the ventral
side just in the same way as does the posterior group. This anterior
group of tubular muscles represents the veno-pericardial muscle of
the segment immediately in front of the opercular, i.e. the metasto-
mal segment, and is the foremost of these veno-pericardial muscles.
Its presence shows that the velar folds, formed as they were by the
breaking down of the septum, are in reality part of two segments,
viz. the opercular and the metastomal, which have fused together
in their basal parts, and by such fusion have caused the inter -relation-
ship between the Vllth and Vth nerves, so apparent in the anatomy
of the vertebrate cranial nerves.

A further piece of evidence that this anterior portion of the velum

THE PROSOMATIC SEGMENTS OF AMMOCCETES 30 1

belongs to the same segment as the lower lip is the fact that in
addition to the tubular muscles a single ordinary striated muscle is
found in the velum which, like the muscles of the lower lip, is
innervated by this same mandibular nerve.

This muscle is attached laterally to the muco-cartilage of the
metastomal bar (s&2) at its junction with the muco-cartilage of the
lower lip, and spreads out into a number of strands which are
attached at intervals along the whole length of the free anterior
edge of the velum. It is the only non-tubular muscle belonging
to the velum, and by its contraction it draws the anterior portions
of the velar folds apart from each other, and so opens the slit
between them, through which the food and mud must pass. Clearly
from its position it does not belong to the original tergo-coxal group
of muscles as do those of the lower lip ; it must have been one of the
intrinsic muscles of the metastoma itself.

This anterior portion of the velar folds affords yet another
striking hint of the correctness of my comparison of the lower lip
segment of Ammoccetes with the chilaria of Limulus or the metas-
toma of Eurypterus ; for the most dorsal anterior portion, which at
its attachment possesses a wedge of muco-cartilage, forms a separate,
well-defined, rounded basal projection marked Ser.ia Fig. 115, and B
in the accompanying Eig. 120. This is that part of the velar folds
which comes together in the middle line and closes the entrance into
the respiratory chamber. The epithelial surface here is most striking
and suggestive, for it is markedly serrated, being covered wuth a
large number of closely-set projections or serree. The serration of
the surface here is of so marked a character that Langerhans con-
sidered this part of the velar folds to act as a masticating organ,
grinding and rasping the food and mud which passed through the
narrow slit. In fact, Langerhans supposed that this portion of the
velum acted in a manner closely resembling the action of the gnatho-
bases of the prosomatic appendages in Limulus or the Eurypteridae.

This suggestion of Langerhans' is surely most significant, con-
sidering that this somewhat separate portion of the velum, to which
he assigns such a function, is in the very place where the gnathite
portion of the metastomal appendages would have been situated if it
were true that the lower lip and anterior portion of the velum of
Ammoccetes were derived from the metastoma.

In addition to this marked serrated edge the whole surface of

102

THE ORIGIN OF VERTEBRATES

the anterior portion of the velum is covered over with a scale-like
or tubercular pattern remarkably like the surface- ornamentation

J>s.br__\__l

Fig. 120. — Ammocoetes cut open in Mid- Ventral Line to show Position of
Velum ; Velar Folds removed on one side.

tr., trabecule; vol., velum; B., anterior gnathic portion of velum; ps. br., pseudo-
branchial groove ; ra2, muscles of lower lip segment ; m3, muscles of thyro-hyoid
segment ; mts, insertion of tubular muscles of velum near thyroid.

seen in many of the members of the ancient group Eurypterida\ In
Fig. 121 I give a picture of this surface-marking of the velum. It
is striking to see that just as in the case of the invertebrate this
marking aud these serroe are formed simply by the cuticular surface
of the epithelial cells ; a surface which, according to Wolff, possibly

contains chitin. The interpretation which I
would give of the velar folds is therefore
as follows : —

They represent the fused basal parts of
the opercular and metastomal appendages,
the gnatho-bases of the latter still retaining
in a reduced degree their rasping surfaces,
because, owing to their position on each side
of the opening into the respiratory chamber
they were still able to manipulate the food as it passed by them
after the closure of the old mouth.

The whole evidence points irresistibly to the conclusion that the
mandibular or velar nerve of the trigemiual does supply a splanchnic

Fig. 121. — Surface View
of Anterior Surface
of Velum.

THE PROSOMATIC SEGMENTS OF AMMOCCETES 303

segment which is, in all respects, comparable with the segments
supplied by the facial, glossopharyngeal, and vagus nerves, except
that it does not possess branchiae. This simply means that the
appendages which these nerves originally supplied were prosomatic,
not mesosomatic, and corresponded, therefore, to the chilarial or
metastomal appendages.

A comparison of the ventral surface of Slimonia, as given in
Fig. 8, p. 27, with that of Ammocoetes (Fig. 119), when the thyroid
gland and lower lip muscles have been exposed to view, enables the
reader to recognize at a glance the correctness of this conclusion.

The Tentacular Segments and the Upper Lip.

Anterior to this metastomal segment, Fig. 116 shows a group of
visceral muscles, mi, and yet again a muco-cartilaginous bar, ski, but,
as already stated, no tubular muscles. These visceral muscles indicate
the presence in front of the lower lip-segment of one or more segments
of the nature of appendages. The muscles in question (mi) are the
muscles of the upper lip, the skeletal elements form a pair of large
bars of muco-cartilage (ski), which start from the termination of the
trabecule, and pass ventralwards to fuse 'with the muco-cartilaginous
plate of the lower lip (Figs. 117 and 118). This large bar forms the
tentacular ridge on each side, and gives small projections of muco-
cartilage into each tentacle. In addition to this tentacular bar, a
special bar of muco-cartilage exists for the fused pair of median
tentacles, the so-called tongue, which extends in the middle line
along the whole length of the lower lip, being separated from the
muco-cartilaginous plate of the lower lip by the muscles of the lower
lip. This tongue bar of muco-cartilage joins with the muco-cartilage
of the lower lip at its junction with the thyroid plate, and also with
the tentacular bar just before the latter joins the muco-cartilaginous
plate of the lower lip. This arrangement of the skeletal tissue
suggests that the pair of tentacles known as the tongue stand in a
category apart from the rest of the tentacles ; a suggestion which is
strongly confirmed by the separate character of its nerve-supply, as
already mentioned.

For three reasons, viz. the separateness both of their nerve-supply
and of their skeletal tissue, and the importance they assume at trans-
formation, this pair of ventral tentacles must, it seems to me, be put

304 THE ORIGIN OF VERTEBRATES

into a separate category from the rest of the tentacles. On the other
hand, the innervation of the rest of the tentacles by a single nerve
which sends off a branch as it passes each one, together with the
concentration of their skeletal elements into a single bar, with pro-
jections into each tentacle, points directly to the conclusion that these
tentacles must be considered as a group, and not singly.

I suggest that these tentacles are the remains of the ectognaths
and endognaths ; the tongue representing the two ectognaths, and
the four tentacles on each side the four pairs of endognaths.

As we see, this method of interpretation attributes segmental
value to the tentacles, a conclusion which is opposed to the general
opinion of morphologists, who regard them as having no special
morphological importance, and certainly no segmental value. On
the other hand, the importance of the pair of ventral tentacles, the
' tongue ' of Bathke, which lie in the mid-line of the lower lip, has
been shown by Kaensche, Bujor, and others, all of whom are
unanimous in asserting that at transformation they are converted
into that large and important organ the piston or tongue of the adult
Petromyzon. It is supposed that the rest of the tentacles vanish
at transformation, being absorbed ; they appear to me rather to take
part in the formation of the sucking-disc, so that I am strongly
inclined to believe that the whole of the remarkable suctorial
apparatus of Petromyzon is derived from the tentacles of Ammocoetes.
In other words, on my view, a conversion of the prosomatic appen-
dages into a suctorial apparatus takes place at transformation, just
as is frequently the case among the Arthropoda.

It is to the arrangement of the muscles that wTe look for evidence
of segmental value. As long as it was possible to look upon these
tentacles as mere sensory feelers round the mouth entrance, it was
natural to deny segmental value to them. Matters are now, how-
ever, totally different since Miss Alcock's discovery of the rudimen-
tary muscles at the base of the tentacles and their development at
transformation. If these muscles represent some of the appendage
muscles belonging to the foremost prosomatic segments just as the
ocular muscles represent the dorso- ventral somatic muscles of those
same segments, then we may expect ultimately to be able to give
as good evidence of segmentation in their case as I have been able
to give in the case of these latter muscles ; for the two sets of muscles
are curiously alike, seeing that the eye-muscles do not develop until

THE PROSOMATIC SEGMENTS OF AMMOCCETES 305

transformation, but throughout the Amnioccetes stage remain in
almost as rudimentary a condition as the tentacular muscles.

Another difficulty with respect to the tentacles is the determina-
tion of the number of them, owing to the fact that in addition
to what may be called well-defined tentacles a lar^e number of
smaller tactile projections are found on the surface of the upper lip,
as is seen in Fig. 115. In the very young condition, 7 or 8 mm. in
length, it is easier to make sure on this point. At this stage they
may be spoken of as arranged in two groups : an anterior small
group and a posterior larger group. The anterior group consists of
a pair of very small tentacles and a very small median tentacle, all
three situated quite dorsally in the front part of the upper lip. The
posterior group, which is separate from the anterior, consists of five
pairs of much larger tentacles, the most ventral pair in the mid-line
ventrally on the lower lip being fused together to form the large
ventral median tentacle or tongue already mentioned. This pair,
according to Shipley, is markedly larger than the others. There are,
therefore, five conspicuous tentacles on each side, and in front of
them a smaller pair and a small median dorsal one. In the very
young condition the accessory projections above-mentioned are not
present, or at all events are not conspicuous, and the tentacles are
also markedly larger in comparison to the size of the animal than
in the older condition, where they have distinctly dwindled.

This posterior group of five conspicuous tentacles is the one which
I suggest represents the four endognaths and one ectognath. What
the significance of the small anterior group is, I know not. It is pos-
sible that the chelicerae are represented here, for they are situated
distinctly anterior to the other group ; I know, however, of no sign of
a markedly separate innervation to these most dorsal tentacles such
as I should have expected to find if they represented the chelicerae.

The muscles of the upper lip, which distinctly belong to the
visceral and not to the somatic musculature, form part of the fore-
most segments, and in these muscles the tentacular nerve reaches its
final destination. From their innervation, then, they must have
belonged to the same appendages as the tentacles supplied by the
tentacular nerve, i.e. to the endognaths. What conclusion can we
form as to the probable origin of the upper lip of Ammoccetes ?
Since the oral chamber was formed by the forward growth of the
metastoma, i.e. the lower lip of Ammoccetes, it follows that the upper

306 THE ORIGIN OF VERTEBRATES

lip is the continuation forwards of the original ventral surface of such
an animal as Limulus or a member of the scorpion group, where there
is no inetastoma, and corresponds to the endostoma, as Holm calls it,
of Eurypterus. This termination of the ventral surface in all these
animals is made up of two parts : (1) Of sternites composing the true
median ventral surface of the body, called by Lankester the pro-
and meso-sternites ; and (2) of the sterno-coxal processes of the fore-
most prosomatic appendages, called in the case of Limulus gnathites,
because they are the main agents in triturating the food previously
to its passage into the mouth. In Limulus, a conjoined pro-meso-
sternite forms the median ventral wall to which the sterno-coxal
processes are attached on each side, and in Phrynus and Mygale a
well-marked pro-sternite and meso-sternite are present, forming the
posterior limit of the olfactory opening. In Buthus and the true
scorpions the sterno-coxal processes of the 2nd, 3rd, and 4th pro-
somatic appendages take part in surrounding the olfactory tubular
passage ; in Thelyphonus only the processes of the 2nd pair of pro-
somatic appendages play such a part, the pro-sternite not being
present (cf. Fig. 97).

Seeing, then, what a large share the sterno-coxal processes of one
or more of these prosomatic appendages plays in the formation of
this endostoma, and seeing also that the nerve which supplies the
upper lip-muscles in Ammocoetes is the same as that supplying the
tentacles which are attached to the upper lip, it appears to me more
probable than not that the muscles in question are the vestiges of
the sterno-coxal muscles. These muscles differ markedly in their
attachments from the muscles of the lower lip, for whereas the latter
resemble the tergo-coxal group in their extreme dorsal attachment,
the former resemble the sterno-coxal group in their attachment to
what corresponds to the endostoma.

This interpretation of the meaning of the transformation process
is in accordance with all the previous evidence both from the side
of the palaeostracan as from the side of the vertebrate, for it signifies
that a dwindling process has taken place in the foremost of the
origiual prosomatic appendages — the chelicerie and the endognaths ;
Avhile, on the contrary, the ectognath and the metastoma have con-
tinued to increase in importance right into the vertebrate stage.
This process is simply a continuation of what was already going on
in the invertebrate stage, for whereas in Enrypterus and other cases

THE PROSOMATIC SEGMENTS OF AMMOCCETES 307

the chelicerge and endognaths had dwindled do.wn to mere tentacles,
the ectognath was the large swimming appendage, and the metas-
toma was on the upward grade from the two insignificant chilaria of
Linmlus.

The transformation of these foremost appendages into a suctorial
apparatus is very common among the arthropods, as is seen in the
transformation of the caterpillar into the butterfly, and it is in
accordance with the evidence that the main mass of that suctorial
apparatus should be formed from appendages corresponding to the
ectognath and metastoma rather than from the four endognaths. In
all probability the nucleus masticatorius of the trigeminal nerve with
its innervation of the great muscles of mastication is evidence of the
continued development of the musculature of these two last pro-
somatic appendages, just as the descending root of the Vth demon-
strates the further disappearance of all that belongs to the foremost
prosomatic appendages. As yet, however, as far as I know, the
musculature of the head-region of Petromyzon has not been brought
into line with that of other vertebrates, and until that comparative
study has been completed it is premature to discuss the exact posi-
tion of the masticating muscles of the higher vertebrates.

The analysis of these tentacular segments belonging to the
trigeminal nerve presents greater difficulties than that of any of the
other cranial segments, owing to the deficiency of our knowledge
of what occurs at transformation. Light is required not only on the
origin of the new muscles but also on the origin of the new and
elaborate cartilages which are newly formed at this time.

Miss Alcock has not yet worked out the origin of all these carti-
lages and muscles, so that we are not yet in a position to analyze
the trigeminal supply in Petromyzon into its component appendage
elements, an analysis which ought ultimately to enable us to deter-
mine from which appendage-muscles the masticating muscles in the
higher vertebrates have arisen. As far as the muscles are con-
cerned, she gives me the following information : —

The tongue-nerve supplies in Ammoccetes the rudimentary
muscles which pass laterally from the base of the large ventral
tentacle to the wall of the throat, and even in Ammoccetes must
possess some power of moving that tentacle.

At transformation these muscles proliferate and develop enor-
mously, and form the bulk of the large basilar muscle which

3o8

THE ORIGIN OF VERTEBRATES

surrounds the throat ventrally and laterally, and is the most bulky
muscle in the suctorial apparatus.

The velar or mandibular nerve supplies in Ammoccetes the
muscles of the lower lip. In Petromyzon it supplies also the
longitudinal muscles of the tongue. The tongue-cartilage first
develops in the region of the median ventral tentacle, and there
the longitudinal tongue-muscles first begin to develop, not from
the rudimentary muscles in the tongue but from those in the
lower lip region.

In Ammoccetes the tentacular nerve supplies the rudimentary
muscles in the tentacles and the muscles of the upper lip. The
latter disappear entirely at transformation, and in Petromyzon the
tentacular nerve supplies the circular, pharyngeal, and annular
muscles, which are derived from the rudimentary tentacular
muscles.

For the convenience of my reader I append here a table showing
my conception of the manner in which the endognathal and ecto-
gnathal segments of the Palaeostracan are represented in Ammoccetes.
It shows well the uniform manner in which all the individual
segmental factors have been fused together to represent the appear-
ance of a single segment (van Wijhe's first segment) in the case of
the four endognathal segments, but have retained their individuality
in the case of the ectognathal segment.

V. Wijhe's
segments.

Eurypterid
segments.

Appei
Eurypterid.

idages.
Ammoccetes

Appendage
nerves.

Skeletal
elements.

Somatic
motor
nerves.

Dorso-

ventral

segmental

muecles.

Coelomic
cavities.

Coxal

glands.

2 \

41
5 '

4 Endo-
gnaths

4 Ten-
tacles

1 Ten-
tacular
to 4
tentacles

1 Ten-
tacular
bar to 4
tentacles

1 Oculo-
motor

supply-
ing 4

muscles

Sup.

inf. int.

rectus

and inf.

oblique

1 Pre-
mandi-

bular
fusion

of 4

1 Pitui-
tary
body;
fusion of
4 coxal
glands.

1 Ecto-
gnath

1 Tongue

1 Tongue
nerve

1 Tongue
bar

1 Troch-
learis

supply-
ing 1

muscle

Sup.
oblique

1 Man-
dibular

THE PROSOMATIC SEGMENTS OF AMMDCCETES 309

The Tubular Muscles.

The only musculature innervated by the trigeminal nerve which
remains for further discussion, consists of those peculiar muscles found
in the velum, known by the name of striated tubular muscles. This
group of muscles has already been referred to in Chapter IV., dealing
with respiration and the origin of the heart.

It is a muscular group of extraordinary interest in seeking an
answer to the question of vertebrate ancestry, for, like the thyroid
gland, it bears all the characteristics of a
survival from a prevertebrate form, which
is especially well marked in Ammoccetes.
I have already suggested in this chapter
that the homologues of these muscles are
represented in Limulus by the veno-peri-
cardial group of muscles. I will now
proceed to deal with the evidence for this
suggestion. H^^S B

The structure of the muscle-fibres is
peculiar and very characteristic, so that
wherever they occur they are easily recog-
nized. Each fibre consists of a core of
granular protoplasm, in the centre of which
the nuclei are arranged in a single row.
This core is surrounded by a margin of
striated fibrillse, as is seen in Fig. 122.
Such a structure is characteristic of various
forms of striated muscle found in various

invertebrates, such as the muscle-fibre of mollusca. It is, as far as
I know, found nowhere in the vertebrate kingdom, except in Amrao-
ccetes. At transformation these muscles entirely disappear, becoming
fattily degenerated and then absorbed.

For all these reasons they bear the stamp of a survival from a
prevertebrate form. This alone would not make this tissue of any
great importance, but when in addition these muscles are found to be
arranged absolutely segmentally throughout the whole of the branchial
region, then this tissue becomes a clue of the highest importance.

As mentioned in Chapter IV., the segmental muscles of respira-
tion consist of the adductor muscle and the two constrictor muscles

Fig. 122. — A

Muscle-fibre
ccetes.

Tubular
of Ammo-

A, portion of fibre seen longi-
tudinally ; B, transverse
section of fibre (osmic pre-
paration) ; tbe black dots

are fat-globules.

3IO THE ORIGIN OF VERTEBRATES

— the striated constrictor and the tubular constrictor. Of these
muscles, both the muscles possessing ordinary striation are attached
to the branchial cartilaginous skeleton, whereas the tubular con-
strictors have nothing to do with the cartilaginous basket-work, but
are attached ventrally in the neighbourhood of the ventral aorta.

These segmental tubular muscles are found also in the velar folds
— the remains of the septum or velum which originally separated
the oral from the respiratory chamber. In the branchial region they
act with the other constrictors as expiratory muscles, forcing the
water out of the respiratory chamber. In the living Ammoecetes,
the velar folds on each side can be seen to move synchronously with
the movements of respiration, contracting at each expiration ; they
thus close the slit by which the oral and respiratory chambers com-
municate, and therefore, in conjunction with the respiratory muscles,
force the water of respiration to flow out through the gill-slits, as
described by Schneider.

These tubular muscles thus form a dorso- ventral system of
muscles essentially connected with respiration ; they belong to each
one of the respiratory segments, and are also found in the velum ;
anterior to this limit they are not to be found. What, then, are these
tubular muscles in the velar folds ? Miss Alcock has worked out
their topography by means of serial sections, and, as already fully
explained, has shown that they form exactly similar dorso-ventral
groups, which belong to the two segments anterior to the purely
branchial segments, i.e. to the facial or hyoid segments and the lower
lip-segment of the trigeminal nerve. If the velar folds could be put
back into their original position as a septum, then the hyoid or facial
group of tubular muscles would take up exactly the same position as
those belonsdnu; to each branchial segment.

The presence of these two so clearly segmental groups of muscles
in the velum — the one belonging to the region of the trigeminal, the
other to the region of the facial — is strong confirmation of my con-
tention that this septum between the oral and respiratory chambers
was caused by the fusion of the last prosomatic and the first meso-
soinatic appendages, represented in Limulus by the chilaria and the
operculum.

Yet another clue to the meaning of these muscles is to be found
in their innervation, which is very extraordinary and unexpected.
Throughout the branchial region the striated muscles of each segment

THE PROSOMATIC SEGMENTS OF AMMOClETES

are strictly supplied by the nerve of that segment, and, as already
described, each segment is as carefully mapped out in its innervation
as it is in any arthropod appendage. One exception occurs to this
orderly, symmetrical arrangement : a nerve arises in connection with
the facial nerve, and passes tailwards throughout the whole of the
branchial region, giving off a branch to each segment as it passes.
This nerve {Br. prof., Fig. 123) is known by the name of the ramus
branchialis profundus of the facial, and its extraordinary course has
always aroused great curiosity in the minds of vertebrate anatomists.
Miss Alcock, by the laborious method of following its course through-
out a complete series of sections, finds that each of the segmental
branches which is given off, passes into the tubular muscles of that
segment (Fig. 124). The tubular muscles which belong to the velum,

r.Ree.VII

n L at VII -X

Me*t. j

n'.%. n-.TLj.

Fig. 123.— Diagram showing the Distribution of the Facial Nerve.
Motor branches, red ; sensory branches, blue.

i.e. those belonging to the lower lip-segment and to the hyoid segments,
receive their innervation from the velar or mandibular nerve, and
belong, therefore, to the trigeminal, not to the facial, system.
The evidence presented by these muscles is as follows :—
In the ancestor of the vertebrate there must have existed a seg-
mentally arranged set of dorso-ventral muscles of peculiar structure,
concerned with respiration, and confined to the mesosomatic segments
and to the last prosomatic segment, yet differing from the other
dorso-ventral muscles of respiration in their innervation and their
attachment.

Interpreting these facts with the aid of my theory of the origin
of vertebrates, and remembering that the homologue of the vertebrate
ventral aorta in such a palseostracan as Limulus is the longitudinal

312

THE ORIGIN OF VERTEBRATES

venous sinus, while the opercular and chilarial segments are respec-
tively the foremost mesosomatic and the last prosomatic segments :
they signify that the palreostracan ancestor must have possessed a
separate set of segmental dorso-ventral muscles confined to the bran-
chial, opercular and chilarial or metastomal segments, which, on the

\L£p.pit
-■M.adi-

-Af con- St*.

Fig. 124. — Diagram constructed from a series of Transverse Sections through
a Branchial Segment,' showing the arrangement and relative positions
of the Cartilage, Muscles, Nerves, and Blood-Vessels.

Nerves coloured red are the motor nerves to the branchial muscles. Nerves coloured
blue are the internal sensory nerves to the diaphragms and the external sensory
nerves to the sense-organs of the lateral line system. Br. cart., branchial
cartilage; M. con. sir., striated constrictor muscles; M. con. tub., tubular
constrictor muscles ; M. add., adductor muscle ; D.A., dorsal aorta ; V.A., ventral
aorta; S., sense-organs on diaphragm ; n. hat., lateral line nerve; X., epibran-
chial ganglia of vagus ; E. br. prof. VII. , ramus brandiialis profundus of facial ;
J.v., jugular vein ; Ep. pit., epithelial pit.

one hand, were respiratory in function, and on the other were attached
to the longitudinal venous sinus. Further, these muscles must all
have received a nerve-supply from the neuromeres belonging to the
chilarial and opercular segments, an unsymmetrical arrangement of
nerves, on the face of it, very unlikely to occur in an arthropod.

THE PROSOMATIC SEGMENTS OF AMMOCCETES 313

Is this prophecy borne out by the examination of Limulus ? In
tlie first place, these muscles were dorso -ventral and segmental, and,
referring back to Chapter VII., Lankester arranges the segmental
dorso- ventral muscles in three groups : (1) The dorso- ventral somatic
muscles ; (2) the dorso-ventral appendage muscles ; and (3) the veno-
pericardial muscles. Of these the first group is represented in the
vertebrate by the muscles which move the eye, the second group by
the striated constrictor and adductor muscles and the muscles for the
lower lip. There is, then, the possibility of the third group for this
system of tubular muscles.

Looking first at the structure of these muscles as previously de-
scribed, so different are they in appearance from the ordinary muscles
of Limulus, that Milne-Edwards, as already stated, called them
" brides transparentes," and did not recognize their muscular cha-
racter, while Blanchard called them in the scorpion, "ligaments
contractils."

Consider their attachment and their function. They are attached
to the longitudinal sinus, according to Lankester's observation, in such
a way that the muscle-fibres form a hollow cone filled with blood ;
when they contract they force this blood towards the gills, and thus
act as accessory or branchial hearts. According to Blanchard, in the
scorpion they contract synchronously with the heart ; according to
Carlson, in Limulus they contract with the respiratory muscles. In
Ammoccetes, where the respiration is effected after the fashion of
Limulus, not of Scorpio, the tubular muscles are respiratory in
function.

Look at their limits. The veno-pericardial muscles in Limulus
are limited by the extent of the heart, they do not extend beyond
the anterior limit of the heart. In Fig. 70 (p. 176) two of these
muscles are seen in front of the branchial region also attached to the
longitudinal venous sinus, although in front of the gill-region. In
Ammoccetes the upper limit of the tubular muscles is the group
found in the velum ; this most anterior group belongs to a region in
front of the branchial region — that of the trigeminal.

Moreover, the supposition that the segmental tubular muscles
belong throughout to the veno-pericardial group gives an adenylate
reason why they do not occur in front of the velum; for, as their
existence is dependent upon the longitudinal collecting sinus in
Limulus and Scorpio, which is represented by the ventral aorta in

3 H THE ORIGIN OF VERTEBRATES

Ammocoetes, they cannot extend beyond its limits. Now, Dohrn
asserts that the ventral aorta terminates in the spiracular artery,
which exists only for a short time ; and, in another place, .speaking
of this same termination of the ventral aorta, he states : " Dass je
eine vorderste Arterie aus den beiden primaren Aesten des Conns
arteriosus hervorgeht, die erste Anlage der Thyroidea umfasst, in der
Mesodermfalte des spateren Velums in die Hohe steigt um in die
Aorta der betreffenden Seite einzumunden." These observations
show that the vessel which in Animoccetes represents the longitudinal
collecting sinus in the Merostomata does not extend further forwards
than the velum, and in consequence the representatives of the veno-
pericardial muscles cannot extend into the segments anterior to the
velum. One of the extraordinary characteristics of these tubular
muscles which distinguishes them from other muscles, but brings them
into close relationship with the veno-pericardial group, is the manner
in which the bundles of muscle-fibres are always found lying freely
in a blood-space; this is clearly seen in the branchial region, but
most strikingly in the velum, the interior of which, apart from its
muco-cartilage, is simply a large lacunar blood-space traversed by
these tubular muscles.

All these reasons point to the same conclusion : the tubular
muscles in Ammoccetes are the successors of the veno-pericardial
system of muscles.

If this is so, then this homology ought to throw light on the
extraordinary innervation of these tubular muscles by the branchialis
profundus branch of the facial nerve and the velar branch of the
trigeminal. We ought, in fact, to find in Limulus a nerve arising
exclusively from the ganglia belonging to the chilarial and opercular
segments, which, instead of being confined to those segments, traverses
the whole branchial region on each side, and gives off a branch to
each branchial segment ; this branch should supply the veno-peri-
cardial muscle of that side.

Patten and Eedenbaugh have traced out the distribution of the
peripheral nerves in Limulus, and have found that from each meso-
somatic ganglion a segmental cardiac nerve arises which passes to
the heart and there joins the cardiac median nerve, or rather the
median heart-ganglion, for this so-called nerve is really a mass of
ganglion-cells. In all the branchial segments the same plan exists,
each cardiac nerve belonging to that neuromere is strictly segmental.

THE PROSOMATIC SEGMENTS OF AMMOCCETES 315

Upon reaching the opercular and chilarial neuromeres an extra-
ordinary exception is found ; the cardiac nerves of these two neuro-
meres are fused together, run dorsally, and then form a single nerve
called the pericardial nerve, which runs outside the pericardium
along the whole length of the mesosomatic region, and gives off a
branch to each of the cardiac nerves of the branchial neuromeres as
it passes them.

This observation of Patten and Bedenbaugh shows that the peri-
cardial nerve of Limulus agrees with the very nerve postulated by
the theory, as far as concerns its origin from the chilarial and
opercular neuromeres, its remarkable course along the whole
branchial region, and its segmental branches to each branchial
segment.

At present the comparison goes no further ; there is no evidence
available to show what is the destination of these segmental branches
of the pericardial nerve, and so far all evidence of their having any
connection with the veno-pericardial muscles is wanting. Carlson,
at my request, endeavoured in the living Limulus to see whether
stimulation of the pericardial nerve caused contraction of the veno-
pericardial muscles, but was unable to find any such effect. On the
contrary, his experimental work indicated that each veno-pericardial
muscle received its motor supply from the corresponding mesosomatic
ganglion. This is not absolutely conclusive, for if, as Blanchard
asserts in the case of the scorpion, a close connection exists between
the action of these muscles and of the heart, it is highly probable
that their innervation conforms to that of the heart. Now Carlson
has shown that this cardiac nerve from the opercular and chilarial
neuromeres is an inhibitory nerve to the heart, while -the segmental
cardiac nerves belonging to the branchial ganglia are the augmentor
nerves of the heart.

His experiments, then, show that the motor nerves of the heart
and of the veno-pericardial muscles run together in the same nerves,
but he says nothing of the inhibitory nerves to the latter muscles.
If they exist and if they are in accordance with those to the heart,
then they ought to run in the pericardial nerve, and would naturally
reach the veno-pericardial muscles by the segmental branches of the
pericardial nerve.

Moreover, inhibitory nerves are, in certain cases, curiously
associated with sensory fibres ; so that the nerve which corresponds

3 16 THE ORIGIN OF VERTEBRATES

to the pericardial nerve, viz. the branchialis profundus of the facial,
may be an inhibitory and sensory nerve, and not motor at all. Miss
Alcock's observations are purely histological ; no physiological
experiments have been made.

At present, then, it does not seem to me possible to say that
Carlson's experiments have disproved any connection of the peri-
cardial nerve with the veno-pericardial muscles. We do not know
what is the destination of its segmental branches ; they may still
supply the veno-pericardial muscles even if they do not cause them
to contract; they certainly do not appear to pass directly into them,
for they pass into the segmental cardiac nerves, and can only reach
the muscles in conjunction with their motor nerves. Such a course
would not be improbable when it is borne in mind how, in the frog,
the augmentor nerves run with the inhibitory along the whole length
of the vagus nerve.

Until further evidence is given both as to the function of the seg-
mental branches of the pericardial nerve in the Limulus, and of the
branchialis profundus in Ammoccetes, it is impossible, I think, to
consider that the phylogenetic origin of these tubular muscles is as
firmly established as is that of most of the other organs already
considered. I must say, my own bias is strongly in favour of looking
upon them as the last trace of the veno-pericardial system of muscles,
a view which is distinctly strengthened by Carlson's statement that
the latter system contracts synchronously with the respiratory move-
ments, for undoubtedly in Ammoccetes their function is entirely
respiratory. Then again, although at present there is no evidence to
connect the pericardial nerve in Limulus with this veno-pericardial
system of muscles, yet it is extraordinarily significant that in such
animals as Limulus and Ammoccetes, in both of which the mesoso-
matic or respiratory region is so markedly segmental, an intrusive
nerve should, in each case, extend through the whole region, giving
off branches to each segment. Still more striking is it that this
nerve should arise from the foremost mesosomatic and the last pro-
somatic neuromeres in Limulus — the opercular and chilarial segments
— precisely the same neuromeres which give origin to the correspond-
ing nerve in Ammoccetes, for according to my theory of the origin of
vertebrates, the nerves which supplied the opercular and metastomal
appendages have become the facial nerve and the lower lip-branch
of the trigeminal nerve.

THE PROSOMATIC SEGMENTS OF AMMOCCETES 317

With the formation of the vertebrate heart from the two longi-
tudinal venous sinuses and the abolition of the dorsal invertebrate
heart, the function of these tubular muscles as branchial hearts was
no longer needed, and their respiratory function alone remained. The
last remnant of this is seen in Ammoccetes, for the ordinary striated
muscles were always more efficient for the respiratory act, and so at
transformation the inferior tubular musculature was got rid of, there
being no longer any need for its continued existence.

The Pal^ostoma, or Old Mouth.

The arrangement of the oral chamber in Ammoccetes is peculiar
among vertebrates, and, upon my theory, is explicable by its
comparison with the accessory oral chamber which apparently
existed in Eurypterus. According to this explanation, the lower lip
of the original vertebrate mouth was formed by the coalescence of
the most posterior pair of the prosomatic appendages — the chilaria ;
from which it follows that the vertebrate mouth was not the original
mouth, but a new structure due to such a formation of the lower lip.

It is very suggestive that the direct following out of the original
working hypothesis should lead to this conclusion, for it is universally
agreed by all morphologists that the present mouth is a new forma-
tion, and Dohrn has argued strongly in favour of the mouth being
formed by the coalescence of a pair of gill-slits. Interpret this in
the language of my theory, and immediately we see, as already
explained, gill-slits must mean in this region the spaces between
appendages which did not carry gills ; the mouth, therefore, was
formed by the coalescence of a pair of appendages to form a lower
lip just as I have pointed out.

Where, then, must we look for the pakeostoma, or original mouth \
Clearly, as already suggested, it was situated at the base of the olfac-
tory passage, and the olfactory passage or nasal tube of Ammoccetes
was originally the tube of the hypophysis, so that the following out
of the theory points directly to the tube of the hypophysis as the
place where the palseostoma must be looked for.

This conclusion is not only not at variance with the opinions of
morphologists, but gives a straightforward, simple explanation why
the palaeostoma was situated in the very place where they are most
inclined to locate it. Thus, if we trace the history of the question,

THE ORIGIN OF VERTEBRATES

Hy.

we see that Dohrn's original view of the comparison of the vertebrate
and the annelid led him to the conception that the vertebrate mouth
was formed by the coalescence of a pair of gill-slits, and that the
original mouth was situated somewhere on the dorsal surface and
opened into the gut by way of the infundibulum and the tube of the
hypophysis. This, also, was Cunningham's view as far as the tube
of the hypophysis was concerned. Beard, in 1888, holding the view
that the vertebrates were derived from annelids which had lost their
supra- oesophageal ganglia, and that, therefore, there was no question
of an oesophageal tube piercing the central nervous system of the
vertebrate, explained the close connection of the infundibulum with
the hypophysis by the comparison of the tube of the hypophysis with

N^fe;^'V'-^JA!;-^'^.!i.,.!jUMIlH...l.i..»l.j

..■^^A;;/.iv.\v.v:--^^A-^^^y;.|-|:1:ij|<jj'i-iV

i/jiVvi.v'i-.w'--. — .:••::•,;•• '.■■■.r^.v.:;.-;:';'1j^ii«J»«M'**^

*v\rt"%rr,iii&Sirttiiiii.ii i '

Fig. 125.— Diagram to show the Meeting op the Four Tubes in such a

Vertebrate as the Lamprey.

Nc, neural canal with its infundibular termination ; Nch., notochord ; Al., alimentary
canal with its anterior diverticulum ; Hy., hypophysial or nasal tube ; Or., oral
chamber closed by septum.

the annelidan mouth, so that the infundibular or so-called nervous
portion was a special nervous innervation for the original throat,
just as Kleinenberg had shown to be the case in many annelids.
Beard therefore called this opening of the hypophysial tube the old
mouth, or palaeostoma. Eecently, in 1893, Kupffer has also put
forward the view that the hypophysial opening is the paheostoma,
basing this view largely upon his observations on Ammoccetes and
Acipenser.

As is seen in Fig. 125, the position of this paheostoma is a very
suggestive one. At this single point in Ammoccetes, four separate
tubes terminate ; here is the end of the notochordal tube, the termina-
tion of the infundibulum, the blind end of the nasal tube or tube

THE PROSOMATIC SEGMENTS OF AMMOCCETES 319

of the hypophysis, and the pre-oral elongation of the alimentary
canal.

It is perfectly simple and easy for the olfactory tube to open into
any one of the other three. By opening into the infundibulum it
reproduces the condition of affairs seen in the scorpion ; by opening
into the gut it produces the actual condition of things seen in
Myxine and other vertebrates ; by opening into the notochordal tube
it would produce a transitional condition between the other two.

The view held by Kupffer is that this nasal tube (tube of the
hypophysis) opened into the anterior diverticulum of the vertebrate
gut, and was for this reason the original mouth-tube ; then a new
mouth was formed, and this connection was closed, being subse-
quently reopened as in Myxine. My view is that this tube
originally opened into the infundibulum, in other words, into the
original gut of the pakeostracan ancestor, and was for this reason
the original mouth-tube, in the same sense as the olfactory passage
of the scorpion may be, and often is, called the mouth-tube. When,
with the breaking through of the septum between the oral and
respiratory chambers, the external opening of the oral chamber
became a new mouth, the old mouth was closed but the olfactory
tube still remained, owing to the importance of the sense of smell.
Subsequently, as in Myxine and the higher vertebrates, it opened
into the pharynx, and so formed the nose of the higher vertebrates.

It is not, to my mind, at all improbable that during the transition
stage, between its connection with the old alimentary canal, as in
Eurypterus or the scorpions, and its blind ending, as in Ammocoetes,
the nasal tube opened into the tube of the notochord. This question
will be discussed later on when the probable significance of the
notochord is considered.

The Pituitary Gland.

Turning back to the comparison of Fig. 106, B, and Fig. 106, C,
which represent respectively an imaginary sagittal section through
an Eurypterus-like animal and through Ammocoetes at a larval
stage, all the points for comparison mentioned on p. 244 have now
been discussed with the exception of the suggested homology
between the coxal glands of the one animal and the pituitary
body of the other.

320 THE ORIGIN OF VERTEBRATES

This latter gland undoubtedly arises posteriorly to the hypophysial
tube, or Eathke's pouch (as it is sometimes called), and, as already
mentioned, is supposed by Kupffer to be formed from the posterior
wall of this pouch. More recently, as pointed out in Haller's paper,
Nusbaum, who has investigated this matter, finds that the glandular
hypophysis is not formed from the walls of Eathke's pouch, but from
the tissue of the rudimentary connection or stalk between the two
premandibular cavities, which becomes closely connected with the
posterior wall of Eathke's pouch, and becoming cut off from the
rest of the premandibular cavity on each side, becomes permanently
a part of the ' Hypophysis Anlage.'

The importance of Nusbauin's investigation consists in this, that
he derives the glandular hypophysis from the connecting stalk
between the two premandibular cavities, and therefore from the
walls of the ventral continuation of this cavity on each side.

This may be expressed as follows : —

The ccelomic cavity, known as the premandibular cavity, divides
into a dorsal and a ventral part ; the walls of the dorsal part give
origin to the somatic muscles belonging to the oculomotor nerve,
while the walls of the ventral part on each side form the connecting
stalk between the two cavities, and give origin to the glandular
hypophysis.

Now, as already pointed out, the premandibular cavity is homo-
logous with the 2nd prosomatic ccelomic cavity of Limulus, and this
2nd prosomatic ccelomic cavity divides, according to Kishinouye, into
a dorsal and a ventral part ; and, further, the walls of this ventral
part form the coxal gland. Both in the vertebrate, then, and in
Limulus, we find a marked glandular tissue in a corresponding
position, and the conclusion is forced upon us that the glandular
hypophysis was originally the coxal gland of the invertebrate an-
cestor. As in all other cases already considered, when the facts of
topographical anatomy, of morphology and of embryology, all com-
bine to the same conclusion as to the derivation of the vertebrate
organ from that of the invertebrate, then there must be also a struc-
tural similarity between the two. What, then, is the nature of the
coxal gland in the scorpions and Limulus ? Lankester's paper gives
us full information on this point as far as the scorpion and Limulus
are concerned, and he shows that the coxal gland of Limulus differs
markedly from that of Scorpio in the size of the cells and in the

THE PROSOMATIC SEGMENTS OF AMMOCCETES 32 1

arrangement of the tubes. In Fig. 126, A, I give a picture of a piece
of the coxal gland of Limulus taken from Lankester's paper.

Turning now to the vertebrate, Bela Haller's paper gives us a
number of pictures of the glandular hypophysis from various verte-
brates, and he especially points out the tubular nature of the gland
and its solidification in the course of development in some cases.
In Fig. 126, B, I give his picture of the gland in Ammoccetes.

The striking likeness between Haller's picture and Lankester's
picture is apparent on the face of it, and shows clearly that the
histological structure of the glands in the two cases confirms the
deductions drawn from their anatomical and morphological positions.

n a

Fig. 126. — A, Section of Coxal Gland op Limulus (from Lankester) ; B,
Section of Pituitary Body of Ammoccetes (from Bela Haller\

n.a., termination of nasal passage.

The sequence of events which gave rise to the pituitary body
of the vertebrate was in all probability somewhat as follows : —

Starting with the excretory glands of the Phyllopoda, known as
shell-glands, which existed almost certainly in the phyllopod Trilo-
bite, we pass to the coxal gland of the Merostomata. Judging from
Limulus, these were coextensive with the coxse of the 2nd, 3rd, 4th,
and 5th locomotor appendages. When these appendages became
reduced in size and purely tactile they were compressed and con-
centrated round the mouth region, forming the endognaths of the
Merostomata ; as a necessary consequence of the concentration of the
coxse of the endognaths, the coxal gland also became concentrated,

32 2 THE ORIGIN OF VERTEBRATES

and took up a situation close against the pharynx, as represented in
Fig. 106, B. When, then, the old mouth closed, and the pharynx
became the saccus vasculosis, the coxal gland remained in close
contact with the saccus vasculosus, and became the pituitary body,
thus giving the reason why there is always so close a connection
between the pituitary body and the infundibular region.

Whatever was the condition of the digestive tracts at the transi-
tion stage between the arthropod and the vertebrate, the original
mouth-opening at the base of the olfactory tube was ultimately
closed. The method of its closure was exceedingly simple and
evident. The membranous cranium was in process of formation by
the extension of the plastron laterally and dorsally ; a slight growth
of the same tissue iu the region of the mouth would suffice to close
it and thus separate the infundibulum from the olfactory tube. As
evidence that such was the method of closure, it is instructive to
see how in Ammoccetes the glandular tissue of the pituitary body
is embedded in and mixed up with the tissue of this cranial wall ; how
the termination of the nasal tube is embedded in this same thickened
mass of the cranial wall — how, in fact, both coxal gland and olfac-
tory tube have become involved in the growth of the tissue of the
plastron, by means of which the mouth was closed.

I have now passed in review the nature of the evidence which
justifies a comparison between the segments supplied by the cranial
nerves of the vertebrate and the prosomatic and mesosomatic segments
of the paheostracan. For the convenience of my readers I have put
these conclusions into tabular form (see p. 323), for all the segments as
far as that supplied by the glossopharyngeal nerves. In both verte-
brate and invertebrate this is a fixed position, for in the former, how-
ever variable may be the number of branchial segments which the
vagus supplies, the second branchial segment is always supplied by a
separate nerve, the glossopharyngeal, and in the latter, though the
number of segments bearing branchiae varies, the minimum number
of such segments (as seen in the Pedipalpi) is never less than two.

Summary.

Tlie general consideration of the evidence of the number of segments, and
their nature in the pro-otic reg-ion of the vertebrate, as given in the last
chapter, is not incompatible with the view that the trigeminal nerve originally

THE PROSOMATIC SEGMENTS OF AMMOCCETES

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324 THE ORIGIN OF VERTEBRATES

supplied seven appendages, which appendages did not parry branchiae, but were
originally used for purposes of locomotion as well as of mastication.

Such appendages clearly no longer exist in the higher vertebrates, the
muscles of mastication only remaining ; but in the earliest fish-forms they must
have existed, as. indeed, is seen in Ptericthys and Bothriolepis. Judging from
all the previous evidence some signs of their existence may reasonably be
expected still to remain in Ammocoetes. Such is indeed the case.

In the adult Petromyzon the trigeminal nerve innervates specially a
massive suctorial apparatus, by means of which it holds on to other fishes, or
to stones in the bottom of the stream. There is here no apparent sign of
appendages. Very great, however, is the difference in the oral chamber of
Ammocoetes ; here there is no sign of any suctorial appai'atus. but instead, a
system of tentacles, together with the remains of the septum or velum, which
originally closed off the oral from the respiratory chamber. These tentacles
are the last remnants of the original foremost prosomatic appendages of the
palfeostracan ancestor. Like the lateral eyes they do not develop until the
transformation comes, but during the whole larval condition their musculature
remains in an embryonic condition, and then from these embryonic muscles
the whole massive musculature of the suctorial apparatus develops ; a sucking
apparatus derived from the modification of appendages, as so frequently occurs
in the arthropods.

The study of Ammocoetes indicates that the velum and lower lip correspond
to the metastoma of the Eurypterid. i.e. the chilaria of Lunulas, while the large
ventral pair of tentacles, called the tongue, correspond to the ectognaths of the
Eurypterids, and probably to the oar-like appendages of Ptericthys and
Bothriolepis. From these two splanchnic segments the suctorial apparatus
in the main arises ; the motor supply of these two segments forms the mass of
the trigeminal nerve-supply, and the nerves supplying them, the velar nerve and
the tongue-nerve, are markedly separate from the rest of the trigeminal nerve.

The rest of the tentacles present much less the sign of independent
segments. In their nerves, their muco- cartilaginous skeleton, and their
rudimentary muscles, they indicate a concentration and amalgamation, such
as might be expected from the concentrated endognaths. The contimiation of
the dwindling process, already initiated in the Eurypterid. would easily result in
the tentacles of Ammocoetes.

The nasal tube of Ammocoetes. which originates in the hypophysial tube,
corresponds absolutely in position and in its original structure, to the olfactory
tube of a scorpion-like animal. From this homology two conclusions of
importance follow: (1) the old mouth, or palaeostoma, of the vertebrate was
situated at the end of this tube, therefore, at the termination of the infundi-
bulum : (2) the upper lip, which by its growth, brings the olfactory tube from
a ventral to a dorsal position, was originally formed by the foremost sternites
or endostoma. or else by the sterno-coxal processes of the second pair of
prosomatic appendag-es of the pala?ostracan ancestor.

In strict accordance with the rest of the comparisons made in this region,
the pituitary body shows by similarity of structure, as well as of position, that
it arose from the coxal glands, which were situated at the base of the four
endognaths.

THE PROS O MA TIC SEGMENTS OF AMMOCCETES 325

One after another, when once the clue has been found, all these mysterious
organs of the vertebrate, such as the pituitary and thyroid glands, fall
harmoniously into their place as the remnants of corresponding important
organs in the palasostraca.

Yet another clue is afforded by the tubular muscles of Ammocoetes. that
strange set of non-vertebrate striated muscles, which are so markedly arranged
in a segmental manner, wliich disappear at transformation, and are never found
in any of the higher vertebrates, for the limits of their distribution correspond
to the veno-pericardial muscles of Limidus.

Their nerve-supply in Ammoccetes is most extraordinary ; for, although
they are segmentally arranged throughout the whole respiratory region, which
is segmentally supplied by the Vllth. IXtli. and Xth nerves, and are found in
front of this region only in one segment, that of the lower lip, which is supplied
by the velar branch of the Vth nerve, yet they are not supplied segmentally,
but only by the velar nerve and a branch of the Vllth, the ramus branchialis
profundus. This latter nerve extends throughout the respiratory region, and
gives off segmental branches to supply these muscles.

It is also a curious coincidence that in such a markedly segmented animal
as Limulus, a nerve— the pericardial nerve— which arises from the nerves of the
chilarial and opercular segments, should pass along the whole respiratory region
and give off branches to each mesosomatic segment. It is strange, to say the
least of it, that the chilarial or metastomal and the opercular segments of
Limulus should, on the theory advocated in this book, correspond to the lower
lip and hyoid segments of the vertebrate. At present the homology suggested
is not complete, for there is no evidence as yet that the veno-pericardial muscles
have anything to do with the pericardial nerve.

CHAPTER X

THE RELATIONSHIP OF AMMOCCETES TO THE MOST
ANCIENT FISHES — THE OSTRACODERMATA

The nose of the Osteostraci. — Comparison of head-shield of Amniocoetes and of
Cephalaspis. — Amniocoetes the only living* representative of these ancient
fishes, — Formation of cranium. — Closure of old mouth. — Rohon's primordial
cranium, — Primordial cranium of Phrynus and Galeodes. — Summary.

The shifting of the orifice of the olfactory passage, which led to the
old mouth, from the ventral to the dorsal side, as seen in the trans-
formation of the ventrally situated hypophysial tube of the young
Amniocoetes, to the dorsally situated nasal tube of the full-grown
Ammoccetes, affords one of the most important clues in the whole of
this story of the origin of vertebrates ; for, if Animoccetes is the
nearest living representative of the first-formed fishes, then we ought
to expect to find that the dorsal head-shield of such fishes is differen-
tiated from that of the contemporary Palasostraca by the presence of
a median frontal opening anterior to the eyes. Conversely, if such
median nasal orifice is found to be a marked characteristic of the
group, in combination with lateral and median eyes, as in Ammoccetes,
then we have strong reasons for interpreting these head-shields by
reference to the head of Ammoccetes.

The oldest known fishes belong to a large group of strange forms
which inhabited the Silurian and Devonian seas, classed together
by Smith Woodward under the name of Ostracodermi. These are
divided into three orders : (1) the Heterostraci, including one family,
the Pteraspidee, to which Pteraspis and Cyathaspis belong ; (2) the
Osteostraci, divisible into two families, the Cephalaspidse and Trema-
taspidas, which include Cephalaspis, Eukeraspis, Auchenaspis or
Thyestes, and Tremataspis ; and (3) the Antiarcha, with one family,
the Astrolepida1, including Astrolepis, Pterichthys, and Bothriolepis.

RELATIONSHIP OF AMMOCCETES TO OSTRACODERMS $2 J

Of these, the first two orders belong to the Upper Silurian, while the
third is Devonian.

The Dorsal Head-Shield of the Osteostraci.

Of the three orders above-named, the Heterostraci and Osteostraci
are the oldest, and among them the Cephalaspidee have afforded the
most numerous and best worked-out specimens. At Eootzikiill, in
the island of CKsel, the form known as Thyestes (Amhenaspis) verru-
cosus is especially plentiful, being found thickly present in among the
masses of Eurypterid remains, which give the name to the deposit.
Of late years this species has been especially worked at by Rohon,
and many beautiful specimens have been figured by him, so that a
considerable advance has been made in our knowledge since Pander,
Eichwald, Huxley, Lankester, and Schmidt studied these most
interesting primitive forms.

All observers agree that the head-region of these fishes was
covered by a dorsal and ventral head-shield, while the body-region
was in most cases unknown, or, as in Eichwald's specimens, and in
the specimens figured in Lankester and Smith Woodward's memoirs,
was made up of segments which were not vertebral in character, but
formed an aponeurotic skeleton, being the hardened aponeuroses
between the body-muscles. This body- skeleton, which possesses its
exact counterpart in Ammoccetes, will be considered more fully when
I discuss the origin of the spinal region of the vertebrate.

Of the two head-shields, ventral and dorsal, the latter is best
known and characterizes the group. It consists of a dorsal plate,
with characteristic horns, which in Thyestes verrucosus (Fig. 128), as
described by Eohon, is composed of two parts, a frontal part and an
occipital part (occ), the occipital part being composed of segments,
and possessing a median ridge — the crista occipitalis. In Lankester's
memoir and in Smith Woodward's catalogue, a large number of known
forms are described and delineated, and we may perhaps say that in
some of the forms, such as Eukcraspis pustulifcrus (Fig. 127, B), the
frontal part of the shield only is capable of preservation as a fossil,
while in Cephalaspis (Fig. 127, A) not only the frontal part but a portion
of the occipital region is preserved, the latter being small in extent
when compared with the occipital region of Auchenaspis (Thyestes).
Finally, in Tremataspis and Didymaspis, the whole of both frontal

THE ORIGIN OF VERTEBRATES

and occipital region is capable of preservation, the line of demarca-
tion between these two regions being well marked in the latter species.

V p-°-v

Fig. 127. — A, Dorsal Head-Shield of Cephalaspis (from Lankester) ; B, Dorsal
Head-Shield of Keraspis (from Lankester).

In the best preserved specimens of all this group of fishes a frontal
median orifice is always present ; it appears in some specimens

obscurely partially divided into
two parts. Perhaps the best
specimen of all was obtained
by Eohon at Rootzikiill, and
is thus described by him : —

The frontal part of the dorsal
head-plate carried (Fig. 128) the
two orbits for the lateral eyes
(I.e.), a marked frontal organ
(fro.), and a median depression
(gl.), to which he gives the
name parietal organ. The oc-
cipital part (oce.) was clearly
segmented, and carried, he
thinks, the branchiae I repro-
duce Eohon's figure of the
frontal organ in Thyestes (Fig.
129) ; he describes it as a
deeply sunk pit, divided in the middle by a slit, which leads deeper
in, he supposes, towards the central nervous system.

Fig. 128. — Dorsal Head-Shield of Thy-
estes (Auchenaspis) verrucosus. (From

ROHON.)

Fro., narial opening ; I.e., lateral eyes ; gl.,
glabellum or plate over brain; Occ, oc-
cipital region.

RELATIONSHIP OF AMMOCCETES TO OSTRACODERMS 329

A similar organ was described by Schmidt in Tremataspis, and
considered by him to be a median nose. Such also is the view of
Jaekel, who points out that a median
pineal eye exists between the two
lateral eyes in this animal, as in all
other of these ancient fishes, so that
this frontal organ does not, as Patten |-
thinks, represent the pineal eye. The
whole of this group of fishes, then, is W

characterized by the following striking ,:-'-i m ?;S ^r '

characteristics : —

1. Two well-marked lateral eyes

,, .,,•, -,. Fig. 129. — Naeial Opening and

near the middle line. T m ^ „ „ m7 .

Lateral Orbits of Thyestes

2. Between the lateral eyes, well- Verrucosus. (From Rohon.)
marked median eyes, very small.

3. In front of the eye-region a median orifice, single.

In addition, behind the eye-region a median plate is always found,
frequently different in structure to the rest of the head-shield, being
harder in texture — the so-called post-orbital plate.

Structure of Head-Shield of Cephalaspis compared with that

of Ammoccetes.

What is the structure of this head-shield ? It has been spoken of
as formed of bone because it possesses cells, being thus unlike the
layers of chitin, which are formed by underlying cells but are not
themselves cellular. At the same time, it is recognized on all sides
that it has no resemblance to bone-structure as seen in fossil remains
of higher vertebrates. The latest and best figure of the structure of
this so-called bone is given in Eohon's paper already referred to. It
is, so he describes, clearly composed of fibrillae and star-shaped cells,
arranged more or less in regular layers, with other sets of similar
cells and fibrilke arranged at right angles to the first set, or at vary-
ing angles. The groundwork of this tissue, in which these cells and
fibrils are embedded, contained calcium salts, and so the whole tissue
was preserved. In places, spaces are found in it, in the deepest
layer large medullary spaces; more superficially, ramifying spaces
which he considers to be vascular, and calls Haversian canals ; the

130

THE ORIGIN OF VERTEBRATES

star-like cells, however, are not arranged concentrically around these
spaces, as in true Haversian canals.

This structure is therefore a calcareous infiltration of a tissue
with cells in it. Where is there anything like it ?

As soon as I saw Rohon's picture (Fig. 130), I was astounded
at its startling resemblance to the structure of muco-cartilage as is
seen in Fig. 131, taken from Ammocoetes. If such muco-cartilage
were infiltrated with lime salts, then the muco-cartilaginous skeleton
of Ammocoetes would be preserved in the fossil condition, and be
comparable with that of Cephalaspis, etc.

^ r5ffiQffll

'"^^^fZk y^ZS\ ^C^ii^

- - - ! / : it -

■ i u(-± --1c

Fig. 130.— Section of a Head-
Plate OF A CEFHALASPID.
(From Rohon.)

Fig. 131. — Section of Muco-
Caetilage from Doesal
Head-Plate of Ammoccetes.

The whole structure is clearly remarkably like Eohon's picture of
a section of the head-plate of a Cephalaspid (Fig. 130). In the latter
case the matrix contains calcium salts, in the former it is composed
of the peculiar homogeneous mucoid tissue which stains so charac-
teristically with thionin. With respect to this calcification, it is
instructive to recall the calcification in the interior of the branchial
cartilages of Limulus, as described in Chapter III., for this example
shows how easy it is to obtain a calcification in this chondro-mucoid
material. With respect to the medullary spaces and smaller spaces in
this tissue, as described by Eohon, I would venture to suggest that they
need not all necessarily indicate blood-vessels, for similar spaces would
appear in the head-shield of Ammocoetes if its muco-cartilage alone

RELATIONSHIP OF AMMOCCETES TO OS TRA CODE RMS **I

were preserved. Of these, some would indicate the position of blood-
vessels, such, for instance, as of the external carotid which traverses
this structure ; but the largest and most internal spaces, resembling
Eohon's medullary spaces, would represent muscles, being filled up
with bundles of the upper lip-muscles.

The Muco-Caktilaginous Head-Shield of Ammoccetes.

The resemblance between the structure of the head- shield of
Thyestes and the muco- cartilage of Ammoccetes, is most valuable,
for muco-cartilage is unique, occurs in no other vertebrate, and every
trace of it vanishes at transformation ; it is essentially a character-
istic of the larval form, and must, therefore, in accordance with all
that has gone before, be the remnant of an ancestral skeletal tissue.
The whole story deduced from the study of Ammoccetes would be
incomplete without some idea of the meaning of this tissue. So
also, as already mentioned, the skeleton of Ammoccetes is incomplete
without taking this tissue into account. It is confined entirely to
the head-region ; no trace of it exists posteriorly to the branchial
basket-work. It consists essentially of dorsal and ventral head-
shields, connected together by the tentacular, metastomal, and thyroid
bars, as already described. The ventral shield forms the muco-carti-
laginous plate of the lower lip and the plate over the thyroid gland,
so that the skeleton ventrally is represented by Fig. 118, B, which
shows how the cartilaginous bars of the branchial basket-work are
separated from each other by this thyroid plate. At transformation,
with the disappearance of this muco-cartilaginous plate, the bars
come together in the middle line, as in the more posterior portion
of the branchial basket-work.

The dorsal head-shield of muco-cartilage covers over the upper
lip, sends a median prolongation over the median pineal eyes and
a lateral prolongation on each side as far as the auditory capsules,
giving the shape of the head-shield of muco-cartilage, as in Fig.
118, C.

Not only then is the structure of the head-shield of a Cephalaspid
remarkably like the muco-cartilage of Ammoccetes, but also its
general distribution strangely resembles that of the Ammoccetes
muco-cartilage.

Now, these head-shields in the Cephalaspida1 and Tremataspidte

332 THE ORIGIN OF VERTEBRATES

vary very much in shape, as is seen by the comparison of Tre-
mataspis .and Auchenaspis with Cephalaspis and Eukeraspis, and
yet, undoubtedly, all these forms belong to a single group, the
Osteostraci.

The conception that Amrnocoetes is the solitary living form allied
to this group affords a clue to the meaning of this variation of
shape, which appears to me to be possible, if not indeed probable.
There is a certain amount of evidence given in the development
of Amrnocoetes which indicates that the branchial region of its
ancestors was covered with plates of muco-cartilage as well as the
prosomatic region.

The evidence is as follows : —

The somatic muscles of Amrnocoetes form a continuous longi-
tudinal sheet of muscles along the length of the body, which are
divided up by connective tissue bands into a series of imperfect
segments or myotomes. This simple muscular sheet can be dissected
off along the whole of the head-region of the animal, with the
exception of the most anterior part, without interfering with the
attachments or arrangements of the splanchnic muscular system in
the least. The reason why this separation can be so easily effected
is to be found in the fact that the two sets of muscles are not
attached to the same fascia. The sheet of fascia to which the
somatic muscles are attached is separated from the fascia which
encloses the branchial cavity by a space (cf. Figs. 63 and 64) filled
with blood-spaces and cells containing fat, in which space is also
situated the cartilaginous branchial basket-work. These branchial
bars are closely connected with the branchial sheet of fascia, and
have no connection with the somatic fascia, their perichondrium
forming part of the former sheet. Upon examination, this space
is seen to lie mainly vascular, the blood-spaces being large and
frequently marked with pigment ; but it also possesses a tissue of its
own, recognized as fat-tissue by all observers. The peculiarity of
the cells of this tissue is their arrangement ; they are elongated cells
arranged at right angles to the plates of fascia, just as the fibres of
the muco-cartilage are largely arranged at right angles to their
limiting plates of perichondrium. These cells do not necessarily
contain fat ; and when they do, the fat is found in the centre of each
cell, and does not push the protoplasm of the cell to the periphery,
as in ordinary fat cells.

RELATIONSHIP OF AMMOCCETES TO OSTRACODERMS 333

m. ph . ._ il

Cor- -

In Fig. 132, B, I give a specimen of this tissue stained by osmic
acid; in Fig. 132, A, I give a drawing of ordinary muco-cartilage
taken from the plate of the lower lip; and in Fig. 133, A, a modifi-
cation of the muco-cartilage taken from the velum, which shows the
formation of a tissue in-
termediate between ordi-
nary muco-cartilage and
this branchial fat-tissue.

Further, in fully-grown
specimens of Ammoccetes,
in the region of undoubted
muco-cartilage, a fatty de-
generation of the cells
frequently appears, to-
gether with an increase
in the blood spaces, — the
precursor, in fact, of the
great change which over- mbr
takes this tissue soon
afterwards, at the time of
transformation, when it is
invaded by blood, and
swept away, except in
those places where new
cartilage is formed. I
conclude, then, that the

tissue of this vascular

Fig. 132. — A, Muco-cartilage op Lower Lip

space was originally muco- (Uc). mphi muscie 0f lower lip; m.sm.,

cartilage, which has de- somatic muscle ; Cor., laminated layer of skin.

~ ~+~;i .1 „„,•„„ 4-V.„ kp^ B, Degenerated Muco-cartilage op Bran-

generated during the lire ' _, _ . , , T1

0 ° chial Region. F., fat layer; P., pigment;

of the AmmOC03teS. The. ^l, blood-space; N., somatic nerve; vi.br.,

fact that in most cases branchial muscle ; m.sm., somatic muscle.

undoubted muco-cartilage

is to be found here and there in this space, is strong confirmation of

the truth of this conclusion.

If this conclusion is correct, we may expect that it would be
confirmed by the embryological history of the tissue, and we ought
to find that in much younger stages a homogeneous tissue of the
same nature as muco-cartilage fills up the spaces in the branchial

in. sra

33-

THE ORIGIN OF VERTEBRATES

region, where in the Ammoccetes only blood and fat-containing cells
are present. For this purpose Shipley kindly allowed me to examine
his series of sections through the embryo at various ages. These
specimens are very instructive, especially those stained by osmic
acid, which preserves the natural thickness of this space better than
other staining methods. At an age when the branchial cartilages are
seen to be formed, when no fat -cells are present, a distinctive tissue
(Fig. 133, B) is plainly visible in the velum and at the base of the
tentacles, in the very position where in the more advanced Ammo-
ccetes muco-cartilage exists. Taking, then, this tissue as our guide,
the specimens show that the space between the skin and the visceral
muscles in which the cartilaginous basket-work lies is filled with a

similar material. At this
. T.^X j stage a sheet of embryonic
~"A tissue occupies the posi-
tion where, later on, blood-
spaces and fat-cells are
found, and this tissue re-
sembles that seen in the
velum and other places
where muco - cartilage is
afterwards found.

I conclude, therefore,
that originally the bran-
chial or mesosomatic re-
gion was covered with a
dorsal plate of muco-cartilage, which carried on its under surface the
dorsal part of the branchial basket-work, and sprang from the central
core of skeletogenous tissue around the notochord ; this plate was
separated from the plate which covered this region ventrally by the
lateral grove in which the gill-slits are situated. The ventral plate
carried on its under surface the ventral part of the branchial basket-
work, and was originally continuous with the plate over the thyroid
gland.

In Fig. 134, A, B, C, the cranial skeleton of Ammoccetes is
represented from the dorsal, ventral, and lateral aspects. The
muco-cartilage is coloured red, the branchial or soft cartilage blue,
and the hard cartilage purple. The degenerated muco-cartilage of
the branchial region is represented as an uncoloured plate, on

A B

Fig. 133. — A, Muco-Cartilage of Velum;
B, Embryonic Muco-Cartilage op Tentacu-
lar Bar.

RELATIONSHIP OF AMMOCCETES TO OSTRACODERMS 335

TIC

Fig. 134. — Skeleton op Head-Region of AjimocosTes. A, Lateral View; B,

Ventral View; C, Dorsal View.

Muco-cartilage, red ; soft cartilage, blue ; hard cartilage, purple. sklt sks, sk3,
skeletal bars ; c.e., position of pineal eye ; na. cart., nasal cartilage ; peel., pedicle ;
cr., cranium; nc., notochord.

36 THE ORIGIN OF VERTEBRATES

which the branchial basket-work stands in relief. If it were re-
stored to its original condition of nmco-cartilage, it would represent
a uniform plate, on the under surface of which the basket-work
would be situated ; and if it were calcified and made solid, the
branchial basket-work would not show at all in these figures.

Is it possible to find the reason why this skeletal covering has
degenerated so early before transformation, and why the thyroid
plate remains intact until transformation ? We see that all that part
which has degenerated is covered over by the somatic muscles, — by,
in fact, muscles which, being innervated by the foremost spinal
nerves, belong naturally to the region immediately following the
branchial. I suggest, therefore, that the original skeletal covering
of muco-cartilage has remained intact only where it has not been
invaded and covered over by somatic muscles, but has been invaded
by blood and undergone the same kind of degenerative change as
overtakes the great mass of this tissue at transformation wherever
the somatic muscles have overgrown it.

The covering somatic muscles in the branchial region form a
dorsal and ventral group, of which the latter is formed in the embryo
much later than the former, the line of separation between the two
groups being the lateral groove, with its row of branchial openings.
This groove ends at the first branchial opening, but the ventral and
dorsal somatic muscles continue further headwards. It is instruc-
tive to see that, although the lateral groove terminates, the separation
between the two groups of muscles is still marked out by a ridge
of muco-cartilage, represented in Fig. 134, A, which terminates
anteriorly in the opercular bar.

Passing now to the prosomatic region, we find that here, too, the
muco-cartilaginous external covering is divisible into a dorsal and
a ventral head-plate, the ventral head-plate being the plate of the
lower lip, and the dorsal head-plate the plate of muco-cartilage
over the front part of the head. The staining reaction with thionin
maps out this dorsal head-plate in a most beautiful manner, and
shows that the whole of the upper lip -region in front of the nasal
orifice is one large plate of muco-cartilage, obscured largely by the
invasion of the crossing muscles of the upper lip, but left pure and
uninvaded all around the nasal orifice, and where the upper and lower
lips come together. In addition to this foremost plate, a median
tongue of muco-cartilage covers over the pineal eye and fills up the

RELATIONSHIP OF AMMOCCETES TO OSTRACODERMS 337

median depression between the two median dorsal somatic muscles.
Also, two lateral cornua pass caudal wards from the main frontal mass
of muco-cartilage over the lateral eyes, forming the well-known wedge
which separates the dorsal and lateral portions of the dorso-lateral
somatic muscle. In fact, similarly to what we find in the branchial
region, the muco-cartilaginous covering can be traced with greater
or less completeness only in those parts which are not covered by
somatic muscles.

In Fig. 134, A, B, C, this striking muco-cartilaginous head-
shield, both dorsal and ventral, is shown. Seeing that the upper lip
wraps round the lower one on each side, and that this most ventral
edge of the upper lip contains muco-cartilage, as is seen in Fig. 117,
the dorsal head-shield of muco-cartilage ought, strictly speaking, to
extend more ventrally in the drawings. I have curtailed it in order
not to interfere with the representation of the lower lip and tentacu-
lar muco-cartilages.

From what has been said, it follows that the past history of the
skeletal covering of the whole head-region of Aminoccetes, both
frontal and occipital, can be conjectured by means of the ontogenetic
history of the foremost myomeres.

Dohrn and all other observers are agreed that during the develop-
ment of this animal a striking forward growth of the foremost somatic
myomeres takes place, so that, as Dohrn puts it, the body-muscula-
ture has extended forwards over the gill-region, and at the same
time the gill-region has extended backwards. It is therefore prob-
able that in the ancestral form the myotomes, innervated by the first
spinal nerves, immediately succeeded the branchial region. Judging
from Animoccetes, the forward growth was at first confined to the
dorsal region, and therefore invaded the dorsal head-plate, the ventral
musculature being distinctly a later growth. With respect to this
dorsal part of the myotomes, the first myotome is originally situated
some distance behind the auditory capsule, and then grows forward
towards the nasal opening; the lateral part, according to Hatschek,
grows forward more quickly than the dorsal part, and splits itself
above and below the eye into a dorso-lateral part, which extends up
to the olfactory capsule, and a ventro-lateral part (m. lateralis capitis
anterior, superior, and inferior), thus giving rise to the characteristic
appearance of the muco-cartilaginous head-shield of Amniocuetes.

According, then, to the extent of the growth of these somatic

03°

THE ORIGIN OF VERTEBRATES

muscles, the shape of the muco-cartilaginous head-shield will vary,
and if it were calcified and then fossilized we should obtain fossil
head-shields of widely differing configuration, although such fossils
might be closely allied to each other. This is just what is found
in this group. Let the muco-cartilage extend over the whole of
the branchial region of Ammoccetes, the resulting head-shield would
be as in Fig. 135, A; the branchial bars below the muco-carti-
laginous shield might or might not be evident, and the line between
the branchial and the trigeminal region might or might not be
indicated. Such a head-shield would closely resemble those of Didy-
maspis and Tremataspis respectively. Now suppose the somatic
musculature to encroach slightly on the branchial region and also

Fig. 135. — Diagrams to show the different shapes of Head-Shields due to
the forward growth of the somatic musculature.

A, Didymaspis ; B, Auchenaspis ; C, Cephalaspis ; D, Ammoccetes.

laterally to the end of the anterior branchial region, then we should
obtain a shape resembling that of Thyestes (Fig. 135, B). Continue
the same process further, the lateral muscle always encroaching
further than the median masses, until the whole or nearly the whole
branchial region is invested, and we get the head-shield of Cephalaspis
(Fig. 135, C) ; further still, that of Keraspis, and yet still further,
that of Ammoccetes (Fig. 135, D).

So close is this similarity, from the comparative point of view,
between the dorsal head-shield of the Osteostraci and the dorsal
cephalic region of Ammoccetes that it justifies us in taking Ammo-
ccetes as the nearest living representative of such types ; it is justifi-
able, therefore, to interpret by means of Ammoccetes the position of
other organs in these forms. First and foremost is the hard plate

RELATIONSHIP OF AMMOCCETES TO OSTRACODERMS 339

known as the post-orbital plate, so invariably found. In Fig. 13-4, C,
I have inserted (cr.) the position of the membranous cranium of
Ammoccetes, and it is immediately evident that the primordial
cranium of the Osteostraci must occupy the exact position indicated
by this median hard plate. For this very reason this median plate
would be harder than the rest in order to afford a better protection
to the brain underneath. This plate, because of its position, may
well receive the same name as the similar plate in the trilobite
and various palseostracans and be called the glabellum.

Evidence of Segmentation in the Head-Shield — Formation

of Cranium.

We may thus conceive the position of the nose, lateral eyes,
median eyes,, and cranium in these old fishes. In addition, other
indications of a segmentation in this head-region have been found.
The most striking of all the specimens hitherto discovered are some
of Thi/estes verrucosus, discovered by Eohon, in which the dorsal
shield has been removed, and so we are able to see what that dorsal
shield covered.

In Fig. 136, I reproduce his drawing of one of his specimens from
the dorsal and lateral aspects. These drawings show that the frontal
part of the shield covered a markedly segmented part of the animal ;
five distinct segments are visible apart from the median most anterior
region. This segmented region is entirely confined to the prosomatic
region, i.e. to the region innervated by the trigeminal nerve. An
indication of similar markings is given in Lankester's figure of
Hukeraspis pustuliferas (see Fig. 127, B), and, indeed, evidence of
a segmentation under the antero-lateral border of the head-shield
is recognized at the present time, not only in the Cephalaspidee, but
also in the Pteraspidse, as was pointed out to me by Smith Woodward
in the specimens at the British Museum. Also, in Cyathaspis, Jaekel
has drawn attention to markings of a similar segmental nature

(Fig- 137).

There seems, then, little doubt but that these primitive fishes
possessed something in this region which was of a segmental character,
and indicated at least .five segments, probably more.

Rohon entitles his discovery ' the segmentation of the primordial
cranium.' It would, I think, be better to call it the segmentation of

34o

THE ORIGIN OF VERTEBRATES

the anterior region of the head, for that is in reality what his figures
show, nut the segmentation of the primordial cranium, which, to judge
from Ammocciites, was confined to the region of the glahellum.

What is the interpretation of this appearance ?

Any segmentation in the head-region must be indicative of segments
belonging to the trigeminal or prosomatic region, or of segments
belonging to the vagus or mesosomatic region. Many palaeontologists,

Fig. 136. — Lateral and Dobsal Views
of the Frontal and Occipital Regions
of the Head-Shield of Thyestes, after
Removal of the Outer Surface. (From
Rohon.)

Fig. 137. — Under Surface of Head-
Shield of Cyathaspis. (From
Jaekel.)

A., lateral eyes ; Ep., mcdiau eyes.

looking upon segmentation as indicative of gills and gill-slits, have
attempted to interpret such markings as branchial segments, regard-
less of their position. As the figures show, they extend in front of
the eyes and reach round to the front middle line, a position which
is simply impossible for gills, but points directly to a segmentation
connected with the trigeminal nerve. Comparison with Ammocu'tes
makes it plain enough that the markings in question are prosomatic
in position, and that the gill-region must be sought for in the place

RELATIONSHIP OF AMMOCCETES TO OSTRACODERMS 34 1

where Schmidt and Rohon located it in Thyestes, viz. the so-called
occipital region.

This discovery of Eohon's is, in my opinion, of immense importance,
for it indicates that, in these early fishes, the prosomatic segmenta-
tion, associated with the trigeminal nerve, was much more well-
marked than in any fishes living in the present clay. Why should
it be more well-marked ? Turning to the pakeostracan, it is very
suggestive to compare the markings on their prosomatic carapace
with these markings. Again and again we find indications of seg-
mentation in these fossils similar to those seen in the ancient fishes.
Thus in Fig. 138 I have put side by side the pakeostracan Bunodes
and the fish Thyestes, both life
size. In the latter I have indicated
Iiohon's segments; in the former the
markings usually seen.

From the evidence of Phrynus,
Mygale, etc., as already pointed out,
such markings in the paheostracan
fossils would indicate the position of
the tergo-coxal muscles of the pro-
somatic appendages, even though
such appendages have not yet been
discovered, and it is significant that
in all these cases there is a distinct
indication of a median plate or
glabellum in addition to the seg-
mental markings. Especially instructive is the evidence of Phrynus,
as is seen by a comparison of Figs. 107 and 108, which shows clearly
that this median plate (glab.) covered the brain-region, a brain-region
which is isolated and protected from the tergo-coxal muscles by the
growth dorsal wards of the flanges of the plastron. In this way an
incipient cranium of a membranous character is formed, which helps
to give attachment to these tergo-coxal muscles. As such cranium
is derived directly from the plastron, it is natural that it should
ultimately become cartilaginous, just as occurs when Ammocotes
becomes Petromyzon and the cartilaginous cranium of the latter
arises from the membranous cranium of the former. In Galeodes
also the growth dorsalwards of the lateral flanges of the plastron to
form an incipient cranium in which the brain lies is very apparent.

Fig. 138.— A, Outline op Thyestes
Verrucosus with Rohon's Seg-
ments indicated ; B, Outline op
Bunodes Lunula with Lateral
Eyes inserted.

Both figures natural size.

342 THE O RIG IX OF VERTEBRATES

I venture, then, to suggest that in the Osteostraci the median
hard plate or glabellum protected a brain which was enclosed in a
membranous cranium, very probably not yet complete in the dorsal
region — certainly not complete if the median pineal eyes so univer-
sally found in these ancient fishes were functional — a cranium derived
from the basal trabeculse, in precisely the same manner as we see it
already in its commencement in Phrynus and other scorpions. With
the completion of this cranium and its conversion into cartilage, and
subsequently into bone, an efficient protection was afforded to the
most vital part of the animal, and thus the hard head-shield of the
Palaeostraca and of the earliest fishes was gradually supplanted by
the protecting bony cranium of the higher vertebrates.

Step by step it is easy to follow in the mind's eye the evolution
of the vertebrate cranium, and because it was evolved direct from
the plastron, the impossibility of resolving it into segments is at
once manifest ; for although the plastron was probably originally
segmented, as Schimkewitsch thinks, all sign of such segmentation
had in all probability ceased, before ever the vertebrates first made
their appearance on the earth.

It follows further, from the comparison here made, that those
antero-lateral markings indicative of segments, found so frequently
in these primitive fishes, must be interpreted as due not to gills but
to aponeuroses, due to the presence of muscles which moved proso-
matic appendages, muscles which arose from the dorsal region in
very much the same position as do the muscles of the lower lip in
Ammocoetes ; the latter, as already argued, represent the tergo-coxal
muscles of the last pair of prosomatic appendages — the chilaria or
metastoma. Such an interpretation of these markings signifies that
the first-formed fishes must have possessed prosomatic appendages of
a more definite character than the tentacles of Ammocoetes, something
intermediate between those of the paiasostracau and Ammocoetes.

For my part I should not be in the least surprised were I to hear
that something of the nature of appendages in this region had been
found, especially in view of the well-known existence of the pair of
appendages in the members of the Asterolepidre — large, oar-like
appendages which may well represent the ectognaths.

RELATIONSHIP OF AMMOCCETES TO OSTRACODERMS 343

The Eelationship of the Ostracoderms.

Of the three groups of fishes — the Heterostraci, the Osteostraci,
and the Antiarcha — the last is Devonian, and therefore the latest
in time of the three, while the earliest is the first group, as both
Pteraspis and Cyathaspis have been found in lower levels of the
Silurian age than any of the Osteostraci, and, indeed, Cyathaspis
has been discovered in Sweden in the lower Silurian. This, the
earliest of all groups of fishes, is confined to two forms only —
Pteraspis and Cyathaspis, — for Scaphaspis is now7 recognized to be
the ventral shield of Pteraspis.

Hitherto a strong tendency has existed in the minds both of the
comparative anatomist and the palaeontologist to look on the elasmo-
branchs as the earliest fishes, and to force, therefore, these strange
forms of fish into the elasmobranch ranks. For this purpose the
same device is often used as has been utilized in order to account
for the existence of the Cyclostomata, viz. that of degeneration. The
evidence I have put forward is very strongly in favour of a con-
nection between the cyclostomes and the cephalaspids, and agrees
therefore with all the rest of the evidence that the jawless fishes
are more ancient than those which bore jaws — the Gnathostomata.

This is no new view. It was urged by Cope, who classified the
Heterostraci, Osteostraci, and Antiarcha under one big group — the
Agnatha — from which subsequently the Gnathostomata arose. Cope's
arguments have not prevailed up to the present time, as is seen in
the writings of Traquair, one of the chief authorities on the subject
in Great Britain. He is still an advocate of the elasmobranch origin
of all these earliest fishes, and claims that the latest discoveries of
the Silurian deposits (Thelodus Paget) and other members of the
Ccelolepidse confirm this view of the question.

This view may be summed up somewhat as follows : —
Cartilaginous jaws would not fossilize, and the Ostracoderms may
have possessed them.

They may have degenerated from elasmobranchs just as the
cyclostomes are supposed to have degenerated.

Seeing that bone succeeds cartilage, the presence of bony shields
in Cephalaspis, etc., indicates that their precursors were cartilaginous,
presumably elasmobranch fishes.

Of these arguments the strongest is based on the supposed bony

344 TIIE ORIGIN OF VERTEBRATES

covering of the Osteostraci, with the consequent supposition that
their ancestors possessed a cartilaginous covering This argument is
entirely upset, if, as I have pointed out, the structure of the cepha-
laspid shield is that of muco-cartilage and not of hone. If these
plates are a calcified muco-cartilage. then the whole argument for
their ancestry from animals with a cartilaginous skeleton falls to the
ground, for muco-cartilage is the precursor not only of hone, but also
of cartilage itself.

The evidence, then, points strongly in favour of Cope's view that
the most primitive fishes were Agnatha, after the fashion of cyclo-
stomes, as is also helieved by Smith Woodward, Bashford Dean, and
Jaekel.

Among living animals, as I have shown, the Limulus is the sole
survivor of the palseostracan type, and Ammoco'tes alone gives a
clue to the nature of the cephalaspid, i.e. the osteostracan fish. Older
than the latter is the heterostracan, Pteraspis, and Cyathaspis. Is
it possible from their structure to obtain any clue as to the actual
passage from the pala'ostracan to the vertebrate ?

Here again, as in the case of the Osteostraci, a relationship to the
elasmobranch has been supposed, for the following reasons : —

The latest discoveries in the Silurian and Devonian deposits have
brought to light strange forms such as Thelodus and Drepanaspis, of
which the latter from the Devonian must, according to Traquair, be
included in the Heterostraci. It possessed, as seen in Fig. 139, large
plates, after the fashion of Pteraspis, and also many smaller ones.

The former, from the upper Silurian, belongs to the CYelolepida?,
and was covered over with shagreen composed of small scutes, after
the fashion of an elasmobranch. Traquair suggests that Thelodus
arose from the original elasmobranch stock; that by the fusion of
scutes such a form as Drepanaspis occurred, and, with still further
fusion, Pteraspis.

There are always two ways of looking at a question, and it seems
to me possible and more prol table to turn the matter round and to
argue that the original condition of the surface-covering was that of
large plates, as in Pteraspis. By the subsequent splitting up of such
plates, Drepanaspis was formed, and later on, by further splitting,
the elasmobranch, Thelodus beincr a stage on the way to the forma-
tion of an elasmobranch, and not a backward stage from the elasmo-
branch towards Pteraspis.

RELATIONSHIP OF AJf.VOCCETES TO OSTRACODERMS 345

This method of looking at the problem seems to me to he more
in consonance with the facts than the reverse; for, as pointed out by
Jaekel, the fishes with large plates are the oldest, and in Cyathaspis,
the very oldest of all, the size of the plates is most conspicuous; he
considers, therefore, this preconceived view that large plates are
formed by the fusion of small ones must give way to the opposite
belief.

So also Eohon, as quoted by Traquair, who, in his first paper
accepted Lankester's view that the ridges of the pteraspidian shield

Fig. 139.— Drepanaspis. Ventral and Dorsal Aspects. (After Lankesteb.)

.4., anus; E., lateral eyes.

were formed by the fusion of a linear arrangement of numbers of
placoid scales, suggests in his second paper that these ridges may
have been the most primitive condition of the dermal skeleton of tbe
vertebrate, out of which, by differentiation, the dermal denticles
(placoid scales) of the selachian, as well as their modifications in the
ganoids, teleosteans, and amphibians, have arisen.

( hie thing is agreed upon on all sides ; no sign of bone-corpuscles
is to be found in this dermal covering of Pteraspis. In the deeper
layers are large spaces, the so-called pulp-cavities leading into
narrow canaliculi, the so-called dentine canals. The structure is

46 THE ORIGIN OF VERTEBRATES

looked upon as similar to that of the pulp and dentine canals of
many fish-scales.

( )n the other hand, this dermal covering of Pteraspis has heen
compared by Patten with the arrangement of the chitinous structure
of certain parts of the external covering of Limulus, a comparison
which to my mind presents a great difficulty. The chitin-layers in
Limulus are external to the epidermal cells, being formed by them ;
the layers in Pteraspis which look like chitin must have been interim!
to the epidermal layer; for each vascular canal which passes from
a pulp-cavity on its way to be distributed into the dentine canals
of the ridge gives off short side branches, which open directly
into the groove between the ridges. If these canals were filled with
blood they could not possibly open directly into the open grooves
between the ridges ; these openings must, therefore, have been covered
over with an epithelial layer which covered over the surface of the
animal, and consequently the chitin-like structure must have been
internal to the epidermis, and not external, as on Patten's view.
The comparison of this structure with the dentine of fish-scales
signifies the same tiling, for in the latter the epidermis is external
to the dentine-plates, the hard skeletal structure is in the position
of the cutis, not of the cuticle.

The position appears to me to be this : the dermal cranial skele-
ton of vertebrates, whether it takes the form of a bonv skull or of
the dorsal plates of a cephalaspid or a pteraspid is, in all cases, not
cuticular, i.e. is not an external formation of the epidermal cells, but
is formed in tissue of the nature of connective tissue underlying the
epidermis. On the contrary, the hard part of the head-carapace of
the pateostracan is an external formation of the epidermal cells.

If, then, this tissue of Pteraspis is not to be looked upon as
chitin, how can we imagine its formation ? It is certainly not bone,
for there are no bone-corpuscles ; it is a very regular laminated
structure resembling in appearance chitin rather than anything else.

As in all cases of difficulty, turn to Ammocoetes and let us see
what clue there is to be found there. The skin of Ammocoetes is
peculiar among vertebrates in many respects. It consists of a number
of epidermal cells, as in Pig. 140, the varying function of which
need not be considered here, covered over with a cuticular layer
which is extraordinarily thick for the cuticle of a vertebrate skin ; this
cuticular layer is perforated with fine canaliculi, through which the

RELATIONSHIP OF AMMOCCETES TO OSTRACODERMS 347

secretion of the underlying cells passes, as is seen in Fig. 140, A and
B. This cuticle corresponds to the chitinous covering of the
arthropod, and like it is perforated with canaliculi, and, according to
Lwoff, possibly contains chitin. The epidermal cells rest on a thick
layer of most striking appearance (Fig. 141), for it resembles, in an
extraordinary degree, when examined superficially, a layer of chitin ;
it is called the laminated layer, and is characterized by the extreme
regularity of the lamina?. This appearance is due, as the observa-
tions of Miss Alcock show, to alternate layers of connective tissue
fibres arranged at right angles to each other, each fibre running a
straight course and possessing its own nucleus. Although the fibres
in each layer are packed close together, they are sufficiently apart
to form with the fibres of the
alternate layers a meshwork
rather than a homogeneous
structure, and thus the surface
view of this layer shows a
regular network of very fine
spaces through which nerve-
fibres and fluid pass. This
layer is easily dissolved in a
solution of hypochlorite of soda,
a fluid which dissolves chitin.
Any one looking at Ammo-
ccetes would say that the only
part of its skin which resembles

chitin is this laminated layer, and therefore the only part of its
skin which would afford an indication of the nature of the
skeleton of Pteraspis is this laminated layer, which belongs to
the cutis, and not to the cuticle. Yet another significant peculiarity
of this layer is its entire disappearance at transformation. Miss
Alcock, in a research not yet published, has shown that this layer
is completely broken up and absorbed at transformation; the cutis
of Petromyzon is formed entirely anew, and no longer presents any
regular laminated character, but resembles rather the sub-epidermal
connective tissue layer of the skin of higher vertebrates. This
laminated layer, then, just like the muco-cartilage, shows, by its
complete disappearance at transformation, its ancestral character.
Very suggestive is the arrangement of the different skeletal

A B

Fig. 140. — Epithelial Cells op Ammo-
ccetes to show the canaliculi in the
Thick Cuticle (B). A, Transverse
Section through the Cuticle.

THE ORIGIN OF VERTEBRATES

tissues in the head-region of Ammoccetes. Fig. 141 represents a
section through the head near the pineal eye. Most internally is a,
a section of the membranous cranium, then comes b, the ran co-
cartilaginous skeleton, then c, the laminated layer, and finally d, the
external cuticle. If in Ammoco'tes we possess an epitome of the
history of the vertebrate, how would these layers be represented in

the past ages, supposing they
could be fossilized ?

The most internal layer a, by
the formation of cartilage and
then bone, represents the great
mass of vertebrate fossils ; the
next layer J>, by a process of
calcification, as previously argued,
represents the head-shield of the
Osteostracan fishes ; while the
cuticular layer d, no longer thin,
is the remnant of the Palseo-
stracan head-carapace. Between
these two layers, b and d, lies the
laminated layer c. Intermediate
to the Paheostracan and the Osteo-
stracan comes the Heterostracan,
with its peculiar head-shield — a
head-shield whose origin is more
easily conceivable as arising from
something of the nature of the
laminated layer than from any
other structure represented in
Ammoccetes.

My present suggestion, then,
is this : the transition from the
skeletal covering of the Paheostracan to that of the highest verte-
brates was brought about by the calcification of successive layers
from without inwards, all of which still remain in Ammocoetes and
show how the external chitinous covering of the arthropod was
gradually replaced by the deep-lying internal bony cranium of the
higher vertebrates.

In Ammoccetes the layer which represents the covering of the

. i

Fig. 141. — Section of Skin and Under-
lying Tissues in the Head-Region
op Ammoccetes.

a, cranial wall ; b, muco-cartilage ; c,
laminated layer ; d, external cuticular
layer.

RELATIONSHIP OF AMMOCCETES TO OSTRACODERMS 349

Paheostracan lias already almost disappeared. At transformation
the layers representing the stage arrived at l»y the Heterostracan
and the Osteostracan disappear ; but the stage representing the
higher vertebrates, far from disappearing, by the formation of carti-
lage reaches a higher stage and prepares the way for the ultimate
stage of all — the formation of the bony cranium.

So much for the evidence as to the nature of the structure of the
head-shield of the Pteraspidse.

It suggests that these fishes were covered anteriorly with armoured
plates derived from the cutis layer of the skin, a layer which was
specially thickened and very vascular, apparently, to enable respi-
ration to be very largely, if not entirely, effected by the surface
of the body. It is difficult to understand how the sea-scorpions
breathed, and it is easy to see how the formation of ventral and
dorsal plates enclosing the mesosomatic appendages may at the outset
have hindered the action of the branchiae. The respiratory chamber,
according to my view, had at first the double function of respiration
and digestion. A new digestive apparatus was the pressing need at
the time ; it would, therefore, be of distinct advantage to remove, as
much as possible, the burden of respiration from this incipient
alimentary canal.

What can be said as to the shape of these ancient forms of
fishes ? Certain parts of them are absolutely known, other parts are
guesswork. They are known to have possessed a dorsal shield, a
ventral shield formerly looked upon as belonging to a separate species,
called Scaphaspis, and a spine attached to the dorsal shield. The
rest of their configuration, as given in Smith Woodward's restoration
(Fig. 142) is guesswork ; the fish-like body with its scales, the hetero-
cercal tail, is based on the most insufficient evidence of something
of the nature of scales having being found near the head-plates.

The dorsal shield is characterized by a pair of lateral eyes
situated on the edge of the shield, not as in Cephalaspis near the
middle line. In the middle line, where the rostrum meets the large
dorsal plate, median eyes were situated. But the slightest sign of
any median single nasal opening, such as is so characteristic of the
head-shield of the Osteostraci and of Ammoccetes has never been
discovered. The olfactory organ must have been situated on the
ventral side as in the larval stage of Ammoccetes, or in the Palaeo-
straca. Many of these head-shields are remarkably well preserved,

35o

THE ORIGIN OF VERTEBRATES

and it is difficult to believe that an olfactory opening would nut be
seen if any such had existed, as it does in Thyestes.

The difficulty of interpreting these types is the difficulty of under-
standing their method of locomotion ; that is largely the reason why
the spine has been placed as if projecting from the back, and a fish-
like body with a heterocercal tail-fin added. If, on the contrary, the

Fig. 142. — Restoration of Pteraspis. (After Smith Woodwakd.)

spine is a terminal tail-spine, then, as far as the fossilized remains
indicate, the animal consisted of a dorsal shield, a ventral shield, and
a tail-spine, to which must be added two apparently lateral pieces
and a few scales. If the animal did not possess a flexible body with
a tail-fin, but terminated in a rigid spike after the fashion of a
Limulus-like animal, then it must have moved by means of

RELATIONSHIP OF AMMOCCETES TO OSTRACODERMS 35 I

appendages. At present we have not sufficient evidence to decide
this question.

That the animal crawled about in the mud by means of free
appendages is by no means an impossible view, seeing how difficult
it is to find the remains of appendages in the fossils of this far-back
time, even when we are sure that they existed. Thus, for many
generations, the appendages of trilobites, which occur in such count
less numbers, and in such great variety of form, were absolutely
unknown, until at last, in consequence of a fortunate infiltration
by pyrites, they were found by Beecher preserved down to the
minutest detail. Even to this day no trace of appendages has been
found in such forms as Hemiaspis, Buuodes, Belinurus, Prestwichia.

The whole question of the evidence of any prosomatic appendages
in these ancient fishes is one of very great interest, and of late years
has been investigated by Patten. It has long been known that
forms such as Pterichthys and Bothriolepis possessed two large, jointed
locomotor appendages, and Patten has lately obtained better speci-
mens of Bothriolepis than have ever been found before, which show
not only the general configuration of the fish, but also the presence
of mandibles or gnathites in the mouth-region resembling those of
an arthropod. These mandibles had been seen before (Smith Wood-
ward), but Patten's specimens are more perfect than any previously
described, and cause him to conclude that these ancient fish were
of the nature of arthropods rather than of vertebrates.

Patten has also been able to obtain some excellent specimens of
the under surface of the head of Tremataspis, which, as evident in
Pig. 143, show the presence of a series of holes, ranging on each side
from the mouth-opening, in a semicircular fashion towards the middle
line. He considers that these openings indicate the attachments of
appendages, in opposition to other observers, such as Jaekel, who look
upon them as gill-slits. To my mind, they are not in the right
position for gill-slits ; they are certainly in a prosomatic rather than
in a mesosomatic position, and I should not be at all surprised if
further research justified Patten's position. So convinced is he of
the presence of appendages in all these old forms, that he considers
them to be arthropods rather than vertebrates, although, at the same
time, he looks upon them as indicating the origin of vertebrates from
arthropods. Here, perhaps, it is advisable to say a few words on
Patten's attitude towards this question.

35^

THE ORIGIN OF VERTEBRATES

Two years after I had put forward my theory of the derivation
of vertebrates from arthropods, Patten published, in the Quarterly
Journal of Microscopical Science, simultaneously with my paper in
that journal, a paper entitled "The Origin of Vertebrates from
Arachnids." In this paper he made no reference to my former
publications, but he made it clear that there was an absolutely
fundamental difference between our treatment of the problem ; for
he took the old view that of necessity there must be a reversal of

surfaces in order that the
internal organs should be
in the same relative positions
in the vertebrate and in the
invertebrate. He simply,
therefore, substituted Arach-
nid for Annelid in the old
theory. Because of this
necessity for the reversal
of surfaces he discarded the
terms dorsal and ventral as
indicative of the surfaces of
an animal, and substituted
haemal and neural, thereby
hopelessly confusing the
issue and making it often
very difficult to understand
his meaning.

He still holds to his
original opinion, and I am
still waiting to find out
when the reversal of sur-
faces took place, for his investigations lead him, as must naturally
be the case, to compare the dorsal (or, as he would call it, the
hsemal) surface of Bothriolepis, of the Cephalaspida-, and of the
Pteraspidse with the dorsal surface of the Palseostraca.

All these ancient fishes are, according to him, still in the arthro-
pod stage, have not yet turned over, though in a peculiarly unscien-
tific manner he argues elaborately that they must have swum on
their back rather than on their front, and so indicated the coming
reversal. Because they were arthropods they cannot have had a

Fig. 143. — Under-Surface of Head-Eegion

in Tbemataspis. (After Patten.)

RELATIONSHIP OF AMMOCCETES TO OSTRACODERMS 353

frontal nose-organ; therefore, Patten looks upon the nose and the
two lateral eyes of the Osteostraci as a complex median eye, regard-
less of the fact that the median eyes already existed.

Every atom of evidence Patten has brought forward, every new
fact he has discovered, confirms my position and makes his still more
hopelessly confused. Keep the animal the right side uppermost, and
the evidence of the rocks confirms the transition from the Pakeo-
stracan to the Cyclostome ; reverse the surfaces, and the attempt to
derive the vertebrate from the palaaostracan becomes so confused and
hopelessly muddled as to throw discredit on any theory of the origin
of vertebrates from arthropods. For my own part, I fully expect
that appendages will be found not only in the Cephalaspidse but also
in the Pteraspidse, and I hope Patten will continue his researches
with increasing success. I feel sure, however, his task will be much
simplified if he abandons his present position and views the question
from my standpoint. -

Summary.

The shifting1 of the nasal tube from a ventral to a dorsal position, as seen
in Ammoccetes, is, perhaps, the most important of all clues in connection with
the comparison of Ammocoetes to the Palfeostracan on the one hand, and to the
Cephalaspid on the other ; for, whereas the exact counterpart of the opening-
of such a tube is always found on the dorsal head-shield in all members of the
latter group, nothing of the kind is ever found on the dorsal carapace of the
former group.

The reason for this difference is made immediately evident in the develop-
ment of Ammocoetes itself, for the olfactory tube originates as a ventral tube —
the tube of the hypophysis — in exactly the same position as the olfactory tube of
the Pala^ostracan. and later on .in its development takes up a dorsal position.

In fact, Ammocoetes in its development indicates how the Palwostracan
head-shield became transformed into that of the Cephalaspid.

In another most important character Ammoccetes indicates its relationship
to the Cephalaspid A3, for it possesses an external skeleton or head-shield composed
of muco-cartilage, which is the exact counterpart of the so-called bony head-
shield of the latter g-roup ; and still more strikingly the structure of the
cephalaspidian head-shield is remarkably like that of muco-cartilage. In the one
case, by the deposition of calcium salts, a hard external skeleton, capable of
being preserved as a fossil, has been formed ; in the other, by the absence of
the calcium salts, a soft chondro-mucoid matrix, in which the characteristic
cells and fibrils are embedded, distinguishes the tissue.

The recognition that the head-shields of these most primitive fishes were
not composed of bone, but of muco-cartilage, the precursor of both cartilage
and bone, immediately clears up in the most satisfactory manner the whole

2 A

354 THE ORIGIN OF VERTEBRATES

question of their derivation from elasmobranch fishes ; for the main argument in
favour of the latter derivation is the exceedingly strong one that bone succeeds
cartilage — not vice versa. — therefore, these forms, since their head-shield is bony,
must have arisen from some other fishes with a cartilaginous skeleton, most
probably of an elasmobranch nature. Seeing, however, that the structure of
their shields resembles muco-cartilage much more closely than bone, and that
Ammocoetes forms a head-shield of muco-cartilage closely resembling theirs,
there is no longer any necessity to derive the jawless fishes from the gnatho-
stomatous ; but. on the contrary, we may look with certainty upon the Agnatha
as the most primitive group from which the others have been derived.

The history of the rocks shows that the group of fishes, Pteraspis and
Oyathaspis, are older than the Cephalaspidre — come, therefore, phylogenetically
between the Palreostraca and the latter group. In this group the head-
shields are of a very different character, without any sign of any structure
comparable with that of bone, and although they possessed both lateral and
median eyes, there is never in any case any trace of a dorsal nasal orifice.
Their olfactory passage, like that of the Palasostraca, must have been ventral.

The remarkable comparison which exists between the head-shields of
Amniocoetes and Cephalaspis, enables us to locate the position of the brain and
cranium of the latter with considerable accuracy, and so to compare the
segmental markings found in many of these fossils with the corresponding
marking's, found either in fossil Pakeostraca or on the head-carapaces of living'
scorpions and spiders, such as Phrynus and Mygale. In all cases the cranial
region was covered with a median plate, often especially hard, which corre-
sponded to the glabellum of the trilobite ; the growth of the cranium can be
traced from its beginnings as the upturned lateral flanges of the plastron to the
membranous cranium of Ammocoetes.

From such a comparison it follows that the segments, found in the antero-
lateral region of the head-shield, were not segments of the cranium, but of parts
beyond the region of the cranium, and from their position must have been
segments supplied by the trig-eminal nerve, and not by the vagus group ;
segments, therefore, which did not indicate gills and gill-slits, but muscles,
innervated by the trigeminal nerve ; muscles which, as indicated by the corre-
sponding markings on the carapace of Phrynus. Mygale, etc., were the tergo-
coxal muscles of the prosomatic appendages.

The discovery of the nature of these appendages in the Pteraspidaj
and Cephalaspida?, as Avell as in the Asterolepida? (Pterichthys and Bothrio-
lepis). is a problem of the future, though in the latter, not only have the
well-known oar-like appendages been long since discovered, but Patten has
recently found specimens of Bothriolepis which throw light on the anterior
masticating gnathite-like appendages which these ancient forms possessed.

CHAPTEB XI

THE EVIDENCE OF THE AUDITORY APPARATUS AND
THE ORGANS OF THE LATERAL LINE

Lateral line organs. — Function of this group of organs. — Poriferous sense-
organs on the appendages in Limulus. — Branchial sense-organs. — Proso-
matic sense organs. — Flabellum. — Its structure and position. — Sense-org*ans
of mandibles. — Auditory organs of insects and arachnids. — Poriferous
chordotonal organs. — Balancers of Diptera. — Resemblance to organs of
flabellum. — Racquet-organs of Galeodes. — Pectens of scorpions. — Large
size of nerve to all these special sense-org-ans. — Origin of parachordals and
auditory capsule. — Reason why Tilth nerve passes in and out of capsule. —
Evidence of Ammocoetes. — Intrusion of glandular mass round brain into
auditory capsule. — Intrusion of generative and hepatic mass round brain
into base of flabellum. — Summary.

When speaking of the tripartite arrangement of the cranial nerves,
an arrangement which gave the clue to the meaning of the cranial
segments, I spoke of the trigeminal as supplying the sensory nerves
to the skin in the head-region, and I compared this dorsal system
of afferent nerves to the system of epimeral nerves in Limulus which
supply the prosomatic and mesosomatic carapaces of Limulus with
sensory fibres. I compared the ventral system of eye-muscle nerves
with the system of nerves supplying the segmental dorso-ventral
somatic muscles of the prosomatic region, and I compared the lateral
system of mixed nerves with the nerves supplying the prosomatic
and mesosomatic appendages of Limulus. I compared, also, the
optic nerves and the olfactory nerves with the corresponding nerves
in the same invertebrate group. My readers will see at once that one
well-marked group of nerves — the auditory and lateral line system —
has been entirely omitted up to the present, it has not even been
mentioned in the scheme of the cranial segments ; I have purposely
reserved its consideration until now, because the organs these nerves
supply, though situated in the skin, are of such a special character

56 THE 0 RIG IX OF VERTEBRATES

as to form a category by themselves. These nerves cannot be classed
among the afferent nerves of the skin any more than the nerves of the
optic and olfactory apparatus ; they require separate consideration.
A very extensive literature has grown up on the subject of this
system of lateral line sense-organs and their innervation, the outcome
of which is decisively in favour of this system being classed with the
sense-organs supplied by the auditory nerve, so that in endeavouring
to understand the position of the auditory nerve, we must always
bear in mind that any theory as to its origin must apply to the
system of lateral line nerves as well.

Now, although the auditory apparatus is common to all verte-
brates, the lateral line system is not found in any land-dwelling
animals ; it belongs essentially to the fishes, and is, therefore, an
old system so far as concerns the vertebrate group. Its sense-organs
are arranged along the lateral line of the fish, and, in addition, on
the head-region in three well-marked lines known as the supra-
orbital, infra-orbital, and mandibular line systems. These sense-
organs lie in the skin in a system of canals, and are innervated by
a special nervous system different to that innervating adjacent skin-
areas. The great peculiarity of their innervation consists in the fact
that their nerves all belong to the branchial system of nerves ; no
fibres arise in connection with the trigeminal, but all of them in
connection with the facial, glossopharyngeal and vagus nerves. In
other words, although organs in the skin, their nerve-supply belongs
to the lateral nervous system which supplies splanchnic and not
somatic segments, a system which, according to the theory advanced
in this book, originated in the nerves supplying appendages. The
conclusion, therefore, is that in order to obtain some clue as to the
origin of the sense-organs of this system in the assumed pal^ostracan
ancestor, we must examine the mesosomatic appendages and see
whether they possess any special sense-organs of similar function.

Further, considering that the auditory organ is to be regarded
as a specially developed member of this system, we must especially
look for an exceptionally developed organ in the region supplied
by the auditory nerve.

The question of the origin of this system of lateral line sense-
organs possesses a special interest for all those who attempt to obtain
a solution of the origin of vertebrates, for the upholders of the view
that the vertebrates have descended from annelids have always

THE EVIDENCE OF THE AUDITORY APPARATUS 357

found its strongest support in the similarity of two sets of segmental
organs found in annelids and vertebrates. On the one hand, great
stress was laid upon the similarity of the segmental excretory organs
in the two groups of animals, as will be discussed later ; on the other,
of the similarity of the segmentally arranged lateral sense-organs.

These lateral sense-organs of the annelids have been specially de-
scribed by Eisig in the CapitellidEe, and, according to Lang, " there are
many reasons for considering these lateral organs to be homologous
with the dorsal cirri of the ventral parapodia of other Polychreta, and
in the family of the Glyeeridae we can follow, almost step by step,
the transformation of the cirri into lateral organs." Eisig describes
them in the thoracic prebranchial region as slightly different from
those in the abdominal branchial region ; in the latter region, the
ventral parapodia are gill-bearing, so that these lateral organs are
in the branchial region closely connected with the branchite, just
as is also the case in the vertebrates. It is but a small step from
the gill-bearing ventral parapodia of the annelid to the gill-bearing
appendages of the phyllopod-like protostracan ; so that if we assume
that this is the correct line along which to search for the origin of
the vertebrate auditory apparatus, then, on my theory of the origin
of the vertebrates from a group resembling the Protostraca, it follows
that special sense-organs must have existed either on or in close
connection with the branchial and prebranchial appendages of the
protostracan ancestor of the vertebrates, which would form an inter-
mediate link between the lateral organs of the annelids and the
lateral and auditory organs of the vertebrates.

Further, these special sense-organs could not have been mere
tactile hairs, but must have possessed some special function, and
their structure must have been compatible with that function. Can
we obtain any clear conception of the original function of this whole
system of sense-organs ?

A large amount of experimental work has been done to determine
the function of the lateral line organs in fishes, and they have been
thought at one time or another to be supplementary organs for
equilibration, organs for estimating pressure, etc. The latest experi-
mental work done by Parker points directly to their being organs
for estimating slow vibrations in water in contradistinction to the
cpuicker vibrations constituting sound. He concludes that surface
wave-movements, whether produced by air moving on the water or

58 THE ORIGIN OF VERTEBRATES

solid bodies falling into the water, are accompanied by disturbance*
which are stimuli for the lateral line organs.

One of these segmental organs has become especially important
and exists throughout the whole vertebrate group, whether the animal
lives on land or in water — this is the auditory organ. Throughout,
the auditory organ has a double function — the function of hearing
and the function of equilibration. If, then, this is, as is generally
supposed, a specialized member of the group, it follows that the
less specialized members must possess the commencement of both
these functions, just as the experimental evidence suggests.

In our search, then, for the origin of the auditory organ of verte-
brates, we must look for special organs for the estimation of vibra-
tions and for the maintenance of the equilibrium of the animal,
situated on the appendages, especially the branchial or mesosomatic
appendages ; and, further, we must specially look for an exceptional
development of such segmental organs at the junction of the pro-
somatic and mesosomatic regions.

Throughout this book the evidence which I have put forward
has in all cases pointed to the same conclusion, viz. that the verte-
brate arose by way of the Cephalaspidse from some arthropod, either
belonging to, or closely allied to, the group called Palceostraca, of
which the only living representative is Limulus. If, then, my argu-
ment so far is sound, the appendages of Limulus, both prosomatic
and mesosomatic, ought to possess special sense-organs which are
concerned in equilibration or the apjDreciation of the depth of the
water, or in some modification of such function, and among these
we might expect to find that somewhere at the junction of the pro-
soma and mesosoma such sense-organs were specially developed to
form the beginning of the auditorv organ.

Now, it is a striking fact that the appendages of Limulus do
possess special sense-organs of a remarkable character, which are
clearly not simply tactile. Thus Gegenbaur, as already stated,
has drawn attention to the remarkable branchial sense-organs of
Limulus ; and Patten has pointed out that special organs, which he
considers to be gustatory in function, are present on the mandibles
of the prosomatic appendages. I myself, as mentioned in my address
to the British Association at Liverpool in 1896, searched for some
special sense-organ at the junction of the prosoma and mesosoma,
and was rewarded by finding that that extraordinary adjunct to the

THE EVIDENCE OF THE AUDITORY APPARATUS 359

last locomotor appendage, known as the nabellum, was an elaborate
sense-organ. I now propose to show that all these special sense-
organs are constructed on a somewhat similar plan ; that the structure
of the branchial sense-organs suggests that they are organs for the
estimation of water pressures ; that among air-breathing arthropods
sense-organs, built up on a somewhat similar plan, are universally
found, and are considered to be of the nature of auditory and equi-
libration organs ; and, what is especially of importance, in view of
the fact that the most prominent members of the Palseostraca were
the sea-scorpions, that the remarkable sense-organs of the scorpions
known as the pectens belong apparently to the same group.

The Poriferous Sense-Organs of the Appendages in Limulus.

On all the branchial appendages in Limulus, special sense-organs
are found of a most conspicuous character. They form in the living
animal bluish convex circular patches, the situation of which on the
appendages is shown in Fig. 58. These organs are not found on the
non-branchial operculum. Gegenbaur, who was the first to describe
them, has pointed out how the surface of the organ is closely set
with chitinous goblets shaped as seen in Fig. 144, A, which do not
necessarily project free on the surface, but are extruded on the
slightest pressure. Each goblet fits into a socket in the chitinous
covering, and is apparently easily protruded by variations of pressure
from within. The whole surface of the organ on the appendage is
slightly bulged in the living condition, and the chitin is markedly
softer here than in the surrounding part of the limb. Each of these
organs is surrounded by a thick protection of strongly branching
spines. On the surface of the organ itself no spines are found, only
these goblets, so that the surface-view presents an appearance as in
Fig. 144, B. Each goblet possesses a central pore, which is the
termination of a very fine, very tortuous, very brittle chitinous
tubule (ch.h), which passes from the goblet through the layers of the
chitin into the subjacent tissue. The goblets vary considerably in
size, a few very large ones being scattered here and there. The fine
chitinous tubule is especially conspicuous in connection with these
largest goblets. In the smaller ones there is the same appearance of
a pore and a commencing tube, but I have not been able to trace the
tube through the chitinous layers, as in the case of the larger goblets.

360

THE ORIGIN OF VERTEBRATES

Gegenbaur, in his picture, draws a straight tubule passing from every
goblet among the fine canaliculi of the chitin. He says they are
difficult to see, except in the case of the larger goblets. The tubule
from the larger goblets is most conspicuous, and is in my sections
always tortuous, never straight, as represented by Gegenbaur. A
special branch of the appendage-nerve passes to these organs, and

V '--clit

Fig. 144. — A, A Goblet from
one of the Branchial Sense-
Organs of Limulus (ch.t.,
chitinous tubule) ; B, Surface
View of a Portion of a Bran-
chial Sense-Organ.

Fig. 145.— The Endognaths of Limulus
pushed out of the way on one side
in order to show the position of
the flabellum (fl.) projecting to-
wards the crack between the pro-
somatic and mesosomatic carapaces.

upon the fine branches of this nerve groups of ganglion-cells are seen,
very similar in appearance to the groups described by Patten on the
terminal branches of the nerves which supply the mandibular organs.
At present I can see no mechanism by which the goblets are extruded
or returned into place. In the case of the Capitellidse, Eisig describes
retractor muscles by means of which the lateral sense-organs are

THE EVIDENCE OF THE AUDITORY APPARATUS 36 1

brought below the level of the surface, and he imagines that the pro-
trusion is effected by hydraulic means, by the aid of the vascular
system. In the branchial sense-organs of Limulus there are no
retractor muscles, and it seems to me that both retraction and pro-
trusion must be brought about by alterations of pressure in the
vascular fluids. Certainly the cavity of the organ is very vascular.
If this be so, it seems likely enough that such an organ should be a
very delicate organ for estimating changes in the pressure of the
external medium, for the position of the goblets would depend on
the relation between the pressure of the fluid inside the organ and
that on the surface of the appendage. Whether the chitinous tubule
contains a nerve-terminal or not I am unable to decide from my
specimens, but, judging from Patten's description of the similar
chitinous tubules belonging to the mandibular organs, it is most
highly probable that these tubules also contain a fine terminal
nerve-fibre.

These organs, then, represent segmental branchial sense-organs,
of which it can be said their structure suggests that they may be
pressure-organs ; but the experimental evidence is at present wanting.

Passing now from the branchial to the prosomatic region, the
first thing that struck me was the presence of that most conspicuous
projection at the base of the last locomotor appendage, which is
usually called the flabellum, and has been described by Lankester
as an exopodite of this appendage. It is jointed on to the most basal
portion of the limb (cf. Fig. 155), and projects dorsally from the limb
into the open slit between the prosomatic and mesosomatic carapace,
as is seen in Fig. 145 (_/?.). Of its two surfaces, the undermost is very
convex and the uppermost nearly flat from side to side, the whole
organ being bent, so that when the animal is lying half buried in
sand, entirely covered over by the prosomatic and mesosomatic
carapaces except along this slit between the two, the upper flat or
slightly convex surface of the flabellum is exposed to any movement
of water through this slit, and owing to its possessing a joint, the
direction of the whole organ can be altered to a limited extent. The
whole of this flat upper surface is one large sense-organ of a striking
character, thus forming a great contrast to the convex under surface,
which is remarkably free from tactile spines or special sense-organs.

The nerve going to the flabellum is very large, almost as large
as the nerve to the rest, of the appe adage, and the very large majority

362

THE ORIGIN OF VERTEBRATES

of the nerve-fibres turn towards the flat, uppermost side, where the
sense-organ is situated. Between the nerve-fibres (n.) and the chi-
tinous surface containing the special sense- tubes masses of cells (gl.)
are seen, as in Fig. 146, apparently nerve- cells, which form a broad
border between the nerve-fibres and the pigmented chitinogenous

-,3o

b*%.

=- - h $m> r <*^
■J » \ %^ L/^"^

<=. «"/

Fig. 147. — Section parallel to
the Surface op Flabellum,
showing the porous termi-
NATIONS of the Sense-Organs
and the Arrangement op the
Canaliculi round them.

ch p bfn g1 P ch

Fig. 146. — Section through Flabellum.

ch., chitiuous layers; s.o., sense-organs; sp.,
spike-organ; p., pigment layer; gl., ganglion
cell layer; bl. and n., blood-spaces and nerves.

layer (p.). On the opposite side, nothing of the sort intervenes
between the pigmented layer and the blood- spaces and nerve-fibres
which constitute the central mass of the flabellum.

At present I am inclined to look upon this mass of cells as
constituting a large ganglion, which extends over the whole length
and breadth of the upper surface of the flabellum. At the same

THE EVIDENCE OE THE AUDITORY APPARATUS

time, my preparations are not sufficiently clear to enable me to
trace out the connections of these cells, especially their connections
with the special sense-

organs.

In Fi« 148 I

give

magnified representation
of a section through three
of these fiabellar sense-
organs. As is seen, the
section divides itself into
four zones : (1) the chi-
tinous layer (ch.) ; (2)
the layer of pigment (p.)
and hypodermal cells ;
(3) the layer of ganglion-
cells (gl); and (4) the
layer of nerve fibres (n.)
and blood-spaces (bl.).
The chitinous layer is
composed of the usual
three zones of the Li-
mulus surface — exter-
nally (Fig. 148), a thin
homogeneous layer, fol-
lowed by a thick layer
of chitin (3), in which
the fine vertical tubules
or canaliculi are well
marked ; the external
portion (2) of this layer
is differentiated from the
rest by the presence of
well-marked horizontal
layers in addition to the
canaliculi.

In this chitinous layer

cap

can

clv.t

mmm w

m U ...

: 9 ;*#»

<§\-

Fig. 148. — Sectiok through theek Sbnse-
Organs op Flabellum.

bl, blood-spaces; n., nerve; gl, layer of ganglion-
cells; p., pigment layer; ch., 1, 2, 3, the three
layers of chitin ; ch.t., chitinous tubule in large
tube of sense-organ ; cap., capitellum or swollen
extremity of large tube; can., very fine porous
canals or canaliculi of chitin.

the special sense-organs

are found. They consist of a large tube which passes through all the

layers of the chitin except the thin homogeneous most external layer.

364 THE ORIGIN OF VERTEBRATES

This tube is conical in shape, its-base, which rests on the pigmented
layer, being so large and the organs so crowded together that a section
of the chitin across the base of the tubes gives the appearance of a
honeycomb, the septa of which is all that remains of the chitin.
This large tube narrows down to a thin elongated neck as it passes
through the chitin, and then, at its termination, bulges out again
into an oval swelling (cap.) situated always beneath the homogeneous
most external layer of chitin. Within this tube a fine chitinous
tubule (eh. t.) is situated similar to that seen in the branchial sense-
organs ; it lies apparently free in the tube, not straight, but sinuous,
and it passes right through all the chitinous layers to open at the
surface as a pore ; in the last part of its course, where it passes
through the most external layer (1) of chitin, it lies always at right
angles to the surface.

If the flabellum be stained with methylene blue and acid fuchsin,

then all the canaliculi in the chitin show up as fine red lines, and

present the appearance given in Fig. 148, and it is seen that each

of the terminations of the tubules is surrounded in the homogeneous

layer of chitin by a thick- set circular patch of canaliculi which pass

to the very surface of the chitin, while the canaliculi in other parts

terminate at the commencement of the homogeneous layer and do

not reach the surface. Further, the contents of the oval swelling,

and, indeed, of the tube as a whole, are stained blue, the chitinous

tubule being either unstained or slightly pink in colour. We see,

then, that the chitinous tubule alone reaches the surface, while the

large tube, which contains the tubule, terminates in an oval swelling,

which often presents a folded or wrinkled appearance, as in Fig. 149

(see also Patten's Fig. 1, Plate I.). This terminal bulging of the

tube is reminiscent of the bulging in the chitinous tubes of the lyri-

form organs of the Arachnida, as described by Gaubert, and of the

poriferous chordotonal organs in insects, as described by Graber (see

Fig. 150). This terminal swelling is filled with a homogeneous

refringent mass staining blue with methylene blue, in which I have

seen no trace of a nucleus ; through this the chitinous tubule makes

its way without any sign of bulging on its part. Patten, in his

description of the sense-organs on the mandibles of Limulus, which

are evidently the same in structure as those on the flabellum, refers

to this homogeneous mass as a cuagulum. I doubt whether this

is an adecpaate description ; it appears to me to stain rather more

THE EVIDENCE OF THE AUDITORY APPARATUS 365

readily than a blood- coagulum, yet in the sense of being structure-
less it resembles a coagulum.

The enormous number of these organs crowded together over
the whole flat surface of the nabellum produces a very striking
appearance when viewed on the surface. Such a view presents an
appearance resembling that of the surface-view of the branchial sense-
organs ; in both cases the surface is covered with a great number
of closely set circular plaques, in the centre of each of which is seen
a well-marked pore. The circular plaques in the case of the nabellum
are much smaller than those of the branchial sense-omans, and
clearly are not protrusible as in the latter organs, the appearance as
of a plaque being due to the ring of thickly-set canaliculi round the
central tubule, as already described. When stained with methylene
blue, the surface view of the nabellum under a low power presents
an appearance of innumerable circular blue masses, from each of
which springs a fine bent hair, terminating in a pore at the surface.
"The blue masses are the homogeneous substance (cap.) of the bulgings
seen through the transparent external layer of chitin, and the hairs
are the terminal part of the chitinous tubules. Patten has repre-
sented their appearance in the mandibles in his Fig. 2, Plate I.

The large tubes in the chitin alter in shape according to their
position. Those in the middle of the sensory surface of the flabellum,
in their course through the chitinous layers, are hardly bent at all ;
as they approach the two lateral edges of this surface, their long thin
neck becomes bent more and more, the bending always being directed
towards the middle of the surface (see Fig. 146) ; in this way the
chitinous tubules increase more or less regularly in length from
the centre of the organ to the periphery. The large basal part of
the conical tube contains, besides the chitinous tubule, a number
of nuclei which are confined to this part of the tube ; some of these
nuclei look like those belonging to nerve-fibres, others are apparently
the nuclei of the chitinogenous membrane lining the tube. I have
never seen any sign of nerve-cells in the tube itself.

The only other kind of sense-organ I have found in connection
with these sense-organs are a few spike-like projections, the appear-
ance of which is given in Fig. 149. I have always seen these in the
position given in Fig. 146 (sp.), i.e. at the junction of the surface
which contains the sense-organs and the surface which is free from
them. They are, so far as I have seen, not very numerous ; I have

366

THE ORIGIN OF VERTEBRATES

CA.t

not, however, attempted to examine the whole sense-organ for the
purpose of estimating their number and arrangement.

As is seen in Fig. 149, they possess a fine tubule of the same
character as that of the neighbouring sense-organs, which apparently
terminates at the apex of the projecting spike. They appear to
belong to the same group as the other poriferous sense-organs, and
are of special interest, because in their appearance they form a link
between the latter and the poriferous sense-organs which charac-
terize the pecten of the scorpion (cf. Fig. 152, C).

Such, then, is the structure of this remarkable sense-organ of the
flabellum, as far as I have been able to work it out with the materials

at my disposal. It is
evident that the flabellar
organs, apart from the
spike-organs, are of the
same kind as those de-
scribed by Patten on the
mandibles and chelre of
Limulus, and therefore it
is most probable that the
nerve - terminals in the
chitinous tubules, and
the origin of the latter,
are similar in the two sets
of organs.

These organs, as Patten
has described them, are
situated in lines on the
spines of the mandibles of the prosomatic locomotor appendages,
and are grouped closely together to form a compact sense-organ
on the surface of the inner mandible (Lankester's epicoxite) (i.m.
in Fig. 155), so that a surface- view of the organ here gives
the characteristic appearance of these poriferous sense-patches.
Precisely similar organs are found on the chilaria, which are, in
function at all events, simply isolated mandibles, to use Patten's
terminology.

On the digging appendage (ectognath), as the comparison of
Fig. 155, A and C, shows, the mandibular spines are almost non-
existent, and the inner mandible or epicoxite is not present, so that

Fig. 149.— Spike-Organ op Flabellum.
ch.t., chitinous tubule.

THE EVIDENCE OF THE AUDITORY APPARATUS 367

the special sense-organ of this appendage is represented solely by the
flabellum.

This sketch of the special sense-organs of Lirnulus shows that all
the appendages of Lirnulus possess special sense-organs, with the
exception of the operculum. All these sense-organs are formed on
the same plan, in that they possess a fine chitinous tubule passing-
through the layers of chitin into the underlying hypodermal and
nervous tissues, which terminates on the surface in a pore. The sur-
face of the chitin where these pores are situated is perfectly smooth,
although, in the case of the branchial sense-organs, the goblet-shaped
masses of chitin, each of which contains a pore, are able to be pressed
out beyond the level of the surface.

As to their functions, we unfortunately do not know much that
is definite. Patten considers that he has evidence of a gustatory
function in the case of the mandibular organs, and suggests also a
temperature-sense in the case of some of these organs. The large
organ of the flabellum and the branchial organs he has not taken into
consideration. The situation of these organs puts the suggestion of
any gustatory function, as far as they are concerned, out of the ques-
tion ; and I do not think it probable that such large specialized organs
would exist only for the estimation of temperature, when one sees
how, in the higher animals, the temperature-nerves and the nerves of
common sensation are universally distributed over the body. As
already stated, the structure of the branchial organs seems to me to
point to organs for estimating varying pressures more than anything
else, and I am strongly inclined to look upon the whole set of organs
as the derivatives of the lateral sense-organs of annelids, such as are
described by Eisig in the Capitellidae. This is Patten's opinion with
respect to the mandibular organs ; and from what I have shown,
these organs cannot be separated in type of structure from those of
the flabellum and the branchial sense-organs.

In our search, then, for the origin of the vertebrate auditory organ
in Lirnulus and its allies, we see so far the following indications : —

1. The auditory organ of the vertebrate is regarded as a special
organ belonging to a segmentally arranged set of lateral sense-organs,
whose original function was co-ordination and equilibration.

2. Such a set of segmentally arranged lateral sense-organs is
found in annelids in connection with the dorsal cirri of the ventral
parapodia.

3 68 THE ORIGIN OF VERTEBRATES

3. If, as has been supposed, there is a genetic connection between
(1) and (2) and if, as I suppose, the vertebrates did not arise from
the annelids directly, but from a protostracan group, then it follows
that the lateral sense-organs, one of which gave rise to the auditory
organ, must have been situated on the protostracan appendages.

4. In Limulus, which is the sole surviving representative of the
palseostracan group, such special sense-organs are found on both the
prosomatic and mesosomatic appendages, and therefore may be
expected to give a direct clue to the origin of the vertebrate auditory

organ.

5. Both from its position, its size, and its specialization, the
flabellum, i.e. an organ corresponding to the flabelluin, must be
looked upon as more likely to give a direct clue to the origin of the
auditory organ than the sense-organs of the branchial appendages, or
the so-called gustatory organs of the mandibles.

The Auditoey Organs of Arachnids and Insects.

The difficulty of the investigating these organs consists in the fact
that so little is known about them in those Arthropoda which live in
the water ; the only instance of any organ apparently of the nature
of an auditory organ, is the pair of so-called auditory sacs at the base
of the antenna? in various decapods. We are in a slightly better
position when we turn to the land-living arthropods ; here the pre-
sence of stridulating organs in so many instances carries with it the
necessity of an organ for appreciating sound. It has now been shown
that such stridulating organs are not confined to the Insecta, but are
present also in the scorpion group, and I myself have added to their
number by the discovery of a distinct stridulating apparatus in
various members of the Phrynidre. We may then take it for granted
that arachnids as well as insects hear. Where is the auditory organ ?

Many observers believe that certain surface-organs found uni-
versally among the spiders, to which Gaubert has given the name of
lyriform organs, are auditory in function. His investigations show
that they are universally present on the limbs and pro-meso-sternite
of all spiders ; that they are present singly, not in groups, on the
limbs of Thelyphonus, and that a group of them exists on the second
segment of each limb in the members of the Phrynus tribe. In the
latter case this organ is the most elaborate of all described by him.

THE EVIDENCE OF THE AUDITORY APPARATUS 369

It is especially noticeable that they do not exist in Galeodes or
in the scorpions, but in the former special sense-organs are found in
the shape of the so-called ' racquet-organs,' on the basal segments of
the most posterior pair of appendages, and also, according to Gaubert,
on the extremity of the palps and the first pair of feet, while in the
latter they occur in the shape of the pectens.

This observation of Gaubert suggests that the place of the
lyriform organs in other arachnids is taken in- Galeodes by the
racquet-organs, and in the scorpions by the pectens. Bertkau,
Schimkewitsch, and Wagner, as quoted by Gaubert, all suggest that
the lyriform organs of the arachnids belong to the same group of
sense-organs as the porous chordotonal organs of the Insecta, sense-
organs which have been found in every group of Insecta, and are
generally regarded as auditory organs. Gaubert does not agree with
this, and considers the lyriform organs to be concerned with the
temperature-sense rather than with audition.

The chordotonal organs of insects have been specially studied by
Graber. He divides them into two groups, the poriferous and the
non-poriferous, the former being characterized by the presence of
pores on the surface arranged in groups or lines. These poriferous
chordotonal organs are remarkably constant in position, being found
only at the base of the wings on the subcostal ridge, in marked
contrast to the other group of chordotonal organs which are found
chiefly on the appendages in various regions. The striking character
of this fixity of position of these organs and the universality of their
presence in the whole group, led Graber to the conclusion that in
these poriferous chordotonal organs we are studying a form of
auditory apparatus which characterized the ancestor of the insect-
group. These organs are always well developed on the hind wings,
and in the large group of Diptera the auditory apparatus has usurped
the whole of the function of the wing ; for the balancers or ' halteres,'
as they are called, are the sole representatives of the hind wings, and
they are usually considered to be of the nature of auditory organs.
It is instructive to find that such an auditory organ serves not only
for the purpose of audition, but also as an organ of equilibration ;
thus Lowne gives the evidence of various observers, and confirms it
himself, that removal of the balancers destroys the power of orderly
flight in the animal.

A striking peculiarity of these organs in the Insecta, as described

2 B

37o

THE ORIGIN OF VERTEBRATES

by Graber, is the bulging of the porous canal near its termination
(Fig. 150, C). This bulging is filled with a homogeneous, highly
refractive material, from which, according to Lowne, a chordotonal
thread passes, to be connected with a ganglion-cell and nerve.
This sphere of refractive material he calls the ' capitellum ' of the
chordotonal thread. The presence of this material produces in a
surface view an appearance as of a halo around the terminal placpie
with its central pore ; Graber has attempted to represent this by the
white area round the central area (in Fig. 150, B). A very similar
appearance is presented by the surface view of the flabellum in
those parts where the tube runs straight to the surface, so that the

5.0

i \

Fig. 150 (from Graber). — A, Section op Subcostal Nervure op Hind Wing op
Dytiscus to show patch op Poriferous Organs (s.o.). B, Surface View op
Poriferous Organs ; the White Space round each Organ indicates the
deeper lying Refringent Body which fills the bulging of the Canal
seen in Transverse Section in C.

refractive material which fills the oval bulging shines through the
overlying chitin and appears to surround the terminal placpue with a
translucent halo.

Such a peculiarity must have a very definite meaning, and sug-
gests that the canals in the flabellum of Limulus and in the hind
wings of insects belong to the same class of organ, the chitinous
tubule with its nerve-terminal in the former corresponding to the
chordotonal thread in the latter. One wonders whether this sphere
of refractive material or ' capitellum ' (to use Lowne's phraseology)
is so universally present in order to act as a damper upon the
vibrations of the chordotonal thread in the one case and of the

THE EVIDENCE OF THE AUDITORY APPARATUS 37 1

cliitinous tubule in the other, just as the membrana tcctoria and the
otoliths act in the case of the vertebrate ear.

Patten says that the only organs which seem to him to be compar-
able with the gustatory porous organs of Limulus are the sense-organs
in the extremities of the palps and of the first pair of legs of Galeodes,
as described by Gaubert. I imagine that he was thinking only of
arachnids, for the comparison of his drawings with those of Graber
show what a strong family resemblance exists between the poriferous
sense-organs of Limulus and those of the insects. On the course
of the terminal nerve-fibres, between the nerve-cell and their entrance
into the porous chitinous canal, Graber describes the existence of
rods or scolophores. On the course of the terminal fibres in the
Limulus organ, between the nerve-cells and their entrance into the
porous chitinous canal, Patten describes a spindle-shaped swelling,
containing a number of rod-like thickenings among the fibrils in the
spindle, which present an appearance reminiscent of the rods described
by Graber.

It appears as though a type of sense-organ, characterized by the
presence of pores on the surface and a fine chitinous canal which
opens at these pores, was largely distributed among the Arthropoda.
According to Graber, this kind of organ represents a primitive type
of sense-organ, which was probably concerned with audition and
ecpuilibration, and he expresses surprise that similar organs have not
been discovered among the Crustacea. It is, therefore, a matter of
great interest to find that so ancient a type of animal as Limulus,
closely allied to the primitive crustacean stock, does possess pori-
ferous sense-organs upon its appendages which are directly compar-
able with these poriferous chordotonal organs of the Insecta.

The Pectens of Scorpions.

Among special sense-organs such as those with which I am now
dealing, the pectens of scorpions and the 'racquet-organs' of Gale-
odes must, in all probability, be classed. I have given my reasons
for this conclusion in my former paper.1 At present such reasons
are based entirely upon the structure of the organs ; experimental

1 " The Origin of Vertebrates, deduced from the Study of Ammoccetes." Part X.,
"The Origin of the Auditory Organ : the Meaning of the VHIth Cranial Nerve."
Journ. Anat. and Physiol., vol. 36, 1902.

372

THE ORIGIN OF VERTEBRATES

evidence as to their function is entirely wanting. With respect to the
pectens of the scorpion (Fig. 151), it has been suggested that they
are of the nature of copulatory organs, a suggestion which may be
dismissed without hesitation, for they are not constructed after the
fashion of claspers, but are simply elaborate sense-organs, and, as

such, are found equally in male or female.
The only observer who has hitherto
specially studied the structure of the
sense-organs in the pecten is, as far as
I know, Gaubert, and he describes their
structure together with that of the sense-
organs of the racquets of Galeodes, in
connection with the lyriform organs of
arachnids, as though he recognized a
family resemblance between the three
sets of organs.

The pecten of the scorpions is an
elaborate sense-organ, or rather group of
sense-organs, the special organ being
developed on each tooth of the comb ;
its surface, which is frequently flattened,
being directed backwards and inwards,
when the axis of the pecten is horizontal
at right angles to the length of the body.
The surface view of this part of the tooth
resembles that of the branchial organs or
of the llabellum in Limulus, in that it
is thickly covered with circular patches,
in the centre of which an ill-defined
appearance as of a fine pore is seen. In Fig. 152, B, I give a sketch
of the surface view of a part of the organ.

Transverse sections of a tooth of the comb of Scorpio Europceus
present the appearance given in Fig. 152, A, and show that each
of these circular patches is the surface-view of a goblet-shaped
chitinous organ, Fig. 152, C, from the centre of which a short, some-
what cylindrical chitinous spike projects. Within this spike, and
running through the goblet into the subjacent tissue, is a fine
tubule. The series of goblets gives rise to the appearance of the
circular plaques on the surface-view, while the spike with its tubule

Fig. 151. — Under Surface of
scorfion (androctonus}.

The operculum is marked out
with dots, aud on each side
of it is seeu one of the pec-
tens.

THE EVIDENCE OF THE AUDITORY APPARATUS 373

is the cause of the ill-defined appearance of the central pore, just
as the terminal pore is much less conspicuous on surface-view in the
spike-organs of the flabellurn than in the purely poriferous organs,
no part of which projects beyond the level of the chitinous surface.

The fine tubule is soon lost in the thickened but soft modification
of the chitinous layer (ch.) which is characteristic of the sense-organ ;
at all events, I have not succeeded in tracing it through this layer
with any more success than in the corresponding case of the tubules

B c

Fig. 152.— A, Section through Tooth of Pecten of Scorpion; B, Surface View
of Sense-Organs ; C, Goblet of Sense-Organ more highly magnified.

bl. and n., region of blood-spaces and nerves; gl., ganglion-cell layer; ch., modified

chitinous layer ; 5.0., sense-organ.

belonging to the smaller goblets of the branchial sense-organ of
Limulus already described.

At the base of the modified chitinous layer a series of cells is
seen, many, if not all, of which belong to the chitinogenous layer.
Xext to these is the marked layer of ganglion-cells (gl.), similar to
those seen in the flabellurn of Limulus. The rest of the space in the
section of the tooth is filled up with nerves (n.) and blood-spaces (bl.)
just as in the section, Fig. 146, of the flabellurn of Limulus.

Gaubert does not appear to have seen the goblets at all clearly ;

374

THE ORIGIN OF VERTEBRATES

lie describes them simply as conical eminences, and states that they
" recouvrent nn pore analogue a celni des poils mais plus petit ;
il est rempli par le protoplasma de la conche hypodermiqne."
From the ganglion, according to him, nervous prolongations pass,
which traverse the chitinogenous layer and terminate at the base
of the conical eminences. Each of these prolongations " presente
sur son trajet, mais im peu plus pres du ganglion que de sa termi-
naison peripherique, une cellule nerveuse fusiforme (g.) offrant,
comme celles du ganglion, un gros noyau." He illustrates his

description with the following, Fig. 153,
taken from his paper.

I have not been able to obtain any evi-
dence of a fusiform nerve-cell on the course
of the terminal nerve-fibres as depicted by
him ; fusiform cells there are in plenty, as
depicted in my drawing, but none with a
large nucleus resembling those of the main
ganglion. In no case, either in the flabellum
or in the branchial organs of Limulus, or in
the pecten-organs, have I ever seen a ganglion-
cell within the chitin-layer ; all the nuclei
seen there resemble those of the cells of
the hypodermis or else the elongated nuclei
characteristic of the presence of nerve-fibres.
Gaubert's drawing is a striking one, and I
have looked through my specimens to see
whether there was anything similar, but have
?*., nerve; gl., ganglion. hitherto failed to obtain any definite evidence

of anything of the kind.
I feel, myself, that an exhaustive examination of the structure
and function of the pecten of scorpions ought to be undertaken. At
present I can only draw the attention of my readers to the similarity
of the arrangement of parts, and of the nature of the end-organs, in
the sense-organs of the flabellum of Limulus and of the pecten of
the scorpion. In both cases the special nerve-fibres terminate in
a massive ganglion, situated just below the chitinogenous layer. In
both cases the terminal fibres from these ganglion-cells pass through
the modified chitinous layer to supply end-organs of a striking cha-
racter ; and although the end-organ of the pecten of the scorpion does

Fig. 153 (from Gaubert).
— Section op a Tooth
of Pecten of Scorpion.

THE EVIDENCE OF THE AUDITORY APPARATUS 375

not closely resemble the majority of the end- organs of the flabellum,
yet it does resemble, on the one hand, the isolated poriferous spikes
found on the flabellum (Fig. 149) and, on the other, the poriferous
goblets found on the sense-patches of the branchial appendages of
Limulus (Fig. 144, A), so that a combination of these two end-organs
would give an appearance very closely resembling that of the pecten
of the scorpion.

Finally, the special so-called ' racquet-organs ' of Galeodes, which
are found on the most basal segments of the last pair of prosomatic
appendages, ought also to be considered here. Gaubert has described
their structure, and shown how the nerve-trunk in the handle of the
racquet splits up into a great number of separate bundles, which
spread out fan-shaped to the free edge of the racquet ; each of these
separate bundles supplies a special sense-organ, which terminates
as a conical eminence on the floor of a deep groove, running round
the whole free edge of the racquet. This groove is almost converted
into a canal, owing to the projection of its two sides. Gaubert
imagines that the sense-organs are pushed forward out of the groove
to the exterior by the turgescence of the whole organ ; each of the
nerve-fibres forming a bundle is, according to Gaubert, connected
with a nerve-cell before it reaches its termination.

This sketch of the special sense-organs on the appendages of
Limulus, of the scorpions, of Galeodes, and other arachnids, and their
comparison with the porous chordotonal organs of insects, affords
reason for the belief that we are dealing here with a common group
of organs, which, although their nature is not definitely known,
have largely been accredited with the functions of equilibration and
audition, a group of organs among which the origin of the auditory
organ of vertebrates must be sought for, upon any theory of the
origin of vertebrates from arthropods.

Whenever in any animal these organs are concentrated together
to form a special organ, it is invariably found that the nerve going to
this organ is very large, out of all proportion to the size of the organ,
and also that the nerve possesses, close to its termination in the
organ, large masses of nerve-cells. Thus, although the whole hind
wing in the blow-fly has been reduced to the insignificant balancers
or ' halteres/ yet, as Lowne states, the nerves to them are the largest
in the body.

The pectinal nerve in the scorpion is remarkable for its size, and

376 THE ORIGIN OF VERTEBRATES

so, also, is the nerve to the flabellum in Limulus, while the large size
of the auditory nerve iu the vertebrate, in distinction to the size of
the auditory apparatus, has always aroused the attention of anatomists.

Throughout this book my attention has been especially directed
to both Limulus and the scorpion group in endeavouring to picture to
myself the ancestor of the earliest vertebrates, because the Eury-
pteridse possessed such marked scorpion-like characteristics ; so that in
considering the origin of a special sense-organ, such as the vertebrate
auditory organ near the junction of the prosoma and mesosoma, it
seems to me that the presence of such marked special sense-orgaus as
the flabellum on the one hand and the pecten on the other, must
both be taken into account, even although the former is an adjunct
to a prosomatic appendage, while the latter represents, according to
present ideas, the whole of a mesosomatic appendage.

From the point of view that the VII Ith nerve represents a
segment immediately posterior to that of the Vllth, it is evident
that an organ in the situation of the pecten, immediately posterior
to the operculum, i.e. according to my view, posterior to the segment
originally represented by the Vllth nerve, is more correctly situated
than an organ like the flabellum, which belongs to a segment anterior
to the operculum.

On the other hand, from the point of view of the relationship
between the scorpions and the king-crabs, it is a possibly debatable
question whether the pecten really belongs to a segment posterior to
the operculum. The position of any nerve in a series depends upon its
position of origin in the central nervous system, rather than upon the
position of its peripheral organ. Now, Patten gives two figures of the
brain of the scorpion built up from serial sections. In both he shows
that the main portion of the pectinal nerve arises from a swelling, to
which he gives the name ganglion nodosum. This swelling arises on
each side in close connection with the origin of the most posterior
prosomatic appendage-nerve, according to his drawings, and posteriorly
to such origin he figures a small nerve which he says supplies the
distal parts of the sexual organs. This nerve is the only nerve which
can be called the opercular nerve, and apparently arises posteriorly
to the main part of the pectinal nerve. If this is so, it would
indicate that the pectens arose from sense-organs which were origi-
nally, like the flabella, pre-opercular in position, but have shifted to
a post-opercular position.

THE EVIDENCE OE THE AUDITORY APPARATUS 377

The Origin of the Parachordals and Auditory Cartilaginous

Capsule.

In addition to what I have already said, there is another reason
why a special sense-organ such as the pecten is suggestive of the
origin of the vertebrate auditory organ, in that such a suggestion
gives a clue to the possible origin of the parachordals and auditory
cartilaginous capsules.

In the lower vertebrates the auditory organ is characterized by
being surrounded with a cartilaginous capsule which springs from
a special part of the axial cartilaginous skeleton on each side, known
as the pair of parachordals. The latter, in Ammoccetes, form a
pair of cartilaginous bars, which unite the trabecular bars with the
branchial cartilaginous basket-work. They are recognized throughout
the Vertebrata as distinct from the trabecular bars, thus forming
a separate paired cartilaginous element between the trabecular and
the branchial cartilaginous system, which of itself indicates a position
for the auditory capsule between the prosomatic trabecular and the
mesosomatic branchial cartilaginous system.

The auditory capsule and parachordals when formed are made of
the same kind of cartilage as the trabecular, i.e. of hard cartilage, and
are therefore formed from a gelatin-containing tissue, and not from
muco-cartilage. Judging from the origin already ascribed to the
trabecular, viz. their formation from the great prosomatic entochon-
drite or plastron, this would indicate that a second entochondrite
existed in the ancestor of the vertebrate in the region of the junction
of the prosoma and mesosoma, which was especially connected with
the sense-organ to which the auditory organ owes its origin. This
pair of entochondrites becoming cartilaginous would give origin to
the parachordals, and subsequently to the auditory capsules, their
position being such that the nerve to the operculum would be
surrounded at its origin by the growth of cartilage.

On this line of argument it is very significant to find that
the scorpions do possess a second pair of entochondrites, viz. the
supra-pectinal entochondrites, situated between the nerve-cord and
the pectens, so that if the ancestor of the Cephalaspid was sufficiently
scorpion-like to have possessed a second pair of entochondrites and
at the same time a pair of special sense-organs of the nature either of

7 8 THE ORIGIN OF VERTEBRATES

the pectens or liabella, then the origin of the auditory apparatus
would present no difficulty.

It is also easy to see that the formation of the parachordals from
entochondrites homologous with the supra-pectinal entochondrites,
would give a reason why the Yllth or opercular nerve is involved
with the Vlllth in the formation of the auditory capsule, especially
if the special sense-organ which gave origin to the auditory organ
was originally a pre-opercular sense-organ such as the rlabellum,
which subsequently took up a post-opercular position like that of
the pecten.

The Evidence of Ammoccetes.

As to the auditory apparatus itself, we see that the elaborate
oman for hearing — the cochlea — has been evolved in the vertebrate
phylum itself. In the lowest vertebrates the auditory apparatus
tends more and more to resolve itself into a simple epithelial sac, the
walls of which in places bear auditory hairs projecting into the sac,
and in part form otoliths. Such a simple sac forms the early stage
of the auditory vesicle in Ammoccetes, according to Shipley ; subse-
quently, by a series of foldings and growings together, the chambers of
the ear of the adult Petromyzon, as figured and described by Eetzius,
are formed. Further, we see that throughout the Vertebrata this sac
was originally open to the exterior, the auditory vesicle being first
an open pit, which forms a vesicle by the approximating of its sides,
the last part to close being known as the recess us labyrinthicus ; in
many cases, as in elasmobranchs, this part remains open, or com-
municates with the exterior by means of the ductus endolymphaticios.

Judging, therefore, from the embryological evidence, it would
appear that the auditory organ originated as a special sense-organ,
formed by modified epithelial cells of the surface, which epithelial
surface becoming invaginated, came to line a closed auditory vesicle
under the surface. This special sense-organ was innervated from
a large ganglionic mass of nerve-cells, situated close against the
peripheral sense-cells, the axis-cylinder processes of which formed
the sensory roots of the nerve.

Yet another peculiarity of striking significance is seen in connec-
tion with the auditory organ of Ammocoetes. The opening of the
cartilaginous capsule towards the brain is a large one (Fig. 154), and

THE EVIDENCE OF THE AUDITORY APPARATUS 379

admits the passage not only of the auditory and facial nerves, but
also of a portion of the peculiar tissue which surrounds the brain.
The large cells of this tissue, with their feebly staining nuclei and
the pigment between them, make them quite unmistakable ; and, as
I have already stated, nowhere else in the whole of Ammoccetes is
such a tissue found. When I first noticed these cells within the
auditory capsule, it seemed to me almost impossible that my inter-
pretation of them as the remnant of the generative and hepatic tissue
which surrounds the braiu of animals such as Limulus could be true,
for it seemed too unlikely that a part of the generative system could

vin

. -Au car I

pen hi 1,1 gen

Fig. 154. — Transverse Section through Auditory Capsules and Brain op

Ammoccetes.
Au., auditory organ; VIII, auditory nerve; rjl., ganglion cells of Vlllth nerve;

Au. cart., cartilaginous auditory capsule; gen., cells of old generative tissue

round brain and in auditory capsule ; bl., blood-vessels.

ever have become included in the auditory capsule. Still, they are
undoubtedly there ; and, as already argued with respect to the
substance round the brain, they must represent some pre-existing
tissue which was functional in the ancestor of Ammocretes. If my
interpretation is right, this tissue must be generative and hepatic
tissue, and its presence in the auditory capsule immediately becomes
a most important piece of evidence, for it proves that the auditory
organ must have been originally so situated that a portion of the
generative and hepatic mass surrounding the cephalic region of
the nervous system followed the auditory nerve to the peripheral
sense-organ.

380 THE ORIGIN OF VERTEBRATES

Here there was a test of the truth of my theory ranking second
only to the test of the median eyes ; the strongest possible evidence
of the truth of any theory is given when by its aid new and unex-
pected facts are brought to light. The theory said that in the group
of animals from which the vertebrates arose, a special sense-organ
of the nature of an auditory organ must have existed on the base of
one of the appendages situated at the junction of the prosoma and
mesosoma, and that into this basal part of the appendage a portion
of the cephalic mass of generative and hepatic material must have
made its way in close contiguity to the nerve of the special
organ.

The only living example which nearly approaches the ancient
extinct forms from which, according to the theory, the vertebrates
arose, is Limulus, and, as has already been shown, in this animal, in
the very position postulated by the theory, a large special sense-
organ — the flabellum — exists, which, as already stated, may well
have given rise to a sense-organ concerned with equilibration and
audition. If, further, it be found that a diverticulum of the gene-
rative and hepatic material does accompany the nerve of the
flabellum in the basal part of the appendage, then the evidence
becomes very strong that the auditory organ of Ammoccetes, i.e. of
the ancient Cephalaspids, was derived from an organ homologous
with the flabellum ; that, therefore, the material round the brain of
Ammoccetes was originally generative and hepatic material ; that, in
fact, the whole theory is true, for all the parts of it hang together so
closely that, if one portion is accepted, all the rest must follow. As
pointed out in my address at Liverpool, and at the meeting of the
Philosophical Society at Cambridge, it is a most striking fact that a
mass of the generative and hepatic tissue does accompany the flabellar
nerve into the basal part of this appendage. Into no other appendage
of Limulus is there the slightest sign of any intrusion of the gene-
rative and hepatic masses ; nowhere, except in the auditory capsule,
is there any sign of the peculiar large-celled tissue which surrounds
the brain and upper part of the spinal cord of Ammoccetes. The
actual position of the flabellum on the basal part of the ectognath is
shown in Fig. 155, A, and in Fig. 155, B, I have removed the chitin,
to show the generative and hepatic tissue {gen.) lying beneath.

The reason why, to all appearance, the generative and hepatic
mass penetrates into the basal part of this appendage only is apparent

THE EVIDENCE OF THE AUDITORY APPARATUS 38 1

when we see (as Patten and Redenbaugh have pointed out) to what
part of the appendage the fiahellum in reality belongs.

Patten and Redenbaugh, in their description of the prosomatic
appendages of Limulus, describe the segments of the limbs as (1) the

> > ent

Fig. 155.— A, The Digging Appendage on Ectognath of Limulus; B, The
Middle Protuberance (2) op the Entocoxite opened, to show the
Generative and Hepatic Tissue (gen.) within it ; C, One of the Proso-
matic Locomotor Appendages or Endognaths of Limulus, for comparison
with A.

ft., flabellum; coa;., coxopodite ; ent., entocoxite ; m., mandible ; i.m., inner mandible

or epicoxite.

dactylopodite, (2) the propodite, (3) the mero- and carpo-podites,
(4) the ischiopo'dite, (5) the basipodite, and (6) the coxopodite (cox.
in Fig. 155). Still more basal than the coxopodite is situated the
entocoxite (ent. in Fig. 155), which is composed of three sclerites

382 THE ORIGIN OF VERTEBRATES

or sensory knobs, to use Patten's description. The middle one of
these three sclerites enlarges greatly in the digging appendage, and
grows over the coxopodite to form the Lase from which the flabellum
springs. Thus, as they have pointed out, the flabellum does not
belong to the coxopodite of the appendage, but to the middle sensory
knob of the entocoxite. Upon opening the prosomatic carapace,
it is seen that the cephalic generative and hepatic masses press
closely against the internal surface of the prosomatic carapace and
also of the entocoxite, so that any enlargement of one of the sensory
knobs of the entocoxite would necessarily be filled with a protrusion
of the generative and hepatic masses. This is the reason why the
generative and hepatic material apparently passes into the basal
segment of the ectocmath, and not into that of the endognaths ; it
does not really pass into the coxopodite of the appendage, but into
an enlarged portion of the entocoxite, which can hardly be considered
as truly belonging to the appendage. Kishinouye has stated that
a knob arises in the embryo at the base of each of the prosomatic
locomotor appendages, but that this knob develops only in the last
or digging appendage (ectognath) forming the flabellum. Doubtless
the median sclerites of the entocoxites of the endognaths represent
Kishinouye's undeveloped knobs.

I conclude, therefore, that the flabellum, together with its basal
part, is an adjunct to the appendage rather than a part of it, and
might, therefore, easily remain as a separate and well-developed
entity, even although the appendage itself dwindled down to a
mere tentacle.

The evidence appears to me very strong that the flabellum of
Limulus and the pecten of scorpions are the most likely organs to
give a clue to the origin of the auditory apparatus of vertebrates.
At present both the Eurypterids and Cephalaspids have left us in the
lurch ; in the former there is no sign of either flabellum or pecten ;
in the latter, no sign of any auditory capsule beyond Bohon's dis-
covery of two small apertures situated dorsally on each side of the
middle line in Tremataspis, which he considers to be the termination
of the ductus endolymphaticus on each side. In both cases it is
probable, one might almost say certain, that any such special
sense-organ, if present, was not situated externally, but was sunk
below the surface as in Ammoccetes.

The method by which such a sense-organ, situated externally on

THE EVIDENCE OF THE AUDITORY APPARATUS ^8

JUJ

the surface of the animal, comes phylogenetically to form the lining
wall of an internally situated membranous capsule is given by the
ontogeny of this capsule, which shows step by step how the sense-
organ sinks in and forms a capsule, and finally is entirely removed
from the surface except as regards the ductus endolymphaticus.

Summary.

The special apparatus for hearing is of a very different character from that
for vision or for smell, for its nerve belongs to the infra-infundibular group of
nerves, and not to the supra-infundibular, as do those of the other two special
senses. Of the five special senses the nerves for touch, taste, and hearing, all
belong to the infra-infundibular seg'inental nerve-groups. The invertebrate
origin, then, of the vertebrate auditory nerve must be sought for in the infra-
oesophageal segmental group of nerves, and not in the supra-cesophageal.

The organs supplied by the auditory nerve are only partly for the purpose
of hearing ; there is always present also an apparatus — the semicircular canals
— concerned with equilibration and co-ordination of movements. Such equili-
bration org'ans are not confined to the auditory nerve, but in the water-living
vertebrates are arranged segmentally along the body, forming the organs of the
lateral line in fishes ; the auditory organ is but one of these lateral line organs,
which has been specially developed.

These lateral line organs have been compared to similar segmental organs
found in connection with the appendages in worms, especially the respiratory
appendages. In accordance with this suggestion we see that they are all
innervated from the region of the respiratory nerves — the vag-us, glosso-
pharyngeal, and facial — nerves which originally supplied the respiratory
appendages of the palaeostracan ancestor.

The logical conclusion is that the appendages of the Palaeostraca possessed
special sense-organs concerned with the perception of special vibrations,
especially in the mesosomatic or respiratory region, and that somewhere at the
junction of the prosoma and mesosoma, one of these sense-organs was specially
developed to form the origin of the vertebrate auditory apparatus.

Impressed by this reasoning- I made search for some specially striking
sense-organ at the base of one of the appendages of Liniulus. at the junction of
the prosoma and mesosoma, and was immediately rewarded by the discovery
of the extraordinary nature of the flabellum. which revealed itself as an
elaborate sense-organ supplied with a nerve out of all proportion to its size.
Up to this time no one had the slightest conception that this flabellum was
a special sense-organ ; the discovery of its nature was entirely due to the
logical following- out of the theory of the origin of vertebrates described in
this book.

The structure of this large sense-organ is comparable with that of the
sense-org-ans of the pectens of the scorpion, and of many other organs found
on the appendages of various members of the scorpion group, of arachnids and

-7

84 THE ORIGIN OF VERTEBRATES

other air-breathing arthropods. Many of these organs, such as the lyriform

organs of arachnids, and the ' halteres ' or balancers of the Diptera, are usually
regarded as auditory and equilibration organs.

On all the niesosoniatic appendages of Limulus very remarkable sense-organs
are found, apparently for estimating pressures, which, when the appendages
sank into the body to form with their basal parts the branchial diaphragms of
Ammoccetes. could easily be conceived as remaining- at the surface, and so giving
rise to the lateral line org'ans.

Further confirmation of the view that an organ, such as the flabellum, must
be looked upon as the originator of the vertebrate auditory organ, is afforded by
the extraordinary coincidence that in Limulus a diverticulum of the generative
and hepatic mass accompanies the flabellar nerve into the basal part of the digging
appendage, while in Ammoccetes, accompanying the auditory nerve into the
auditory capsule, there is seen a mass of cells belonging" to that peculiar tissue
which tills up the space between the brain and the cranial walls, and has already,
on other grounds, been homologized with the generative and hepatic masses
which till up the encephalic region of Limidus.

For all these reasons special sense-organs, such as are found in the flabellum
of Limulus and in the pectens of scorpions, may be looked upon as giving
origin to the vertebrate auditory apparatus. In such case it is highly probable
that the parachordals, with the auditory capsules attached, arose from a second
entochondrite of the same nature as the plastron ; a probability which is
increased by the fact that the scorpion does possess a second entochondrite,
which, owing to its special relations to the pecten, is known as the supra-pectinal
entochondrite.

CHAPTER Xir

THE REG 10 X OF THE SPINAL CORD

Difference between cranial and spinal regions. — Absence of lateral root. —
Meristic variation. — Segmentation of cceloni. — Segmental excretory organs.
— Development of nepliric organs ; pronephric, mesonephric, metanepliric.
—Excretory organs of Ampliioxus.— Solenocytes. -Excretory organs of
Branchipus and of Peripatus, appendicular and somatic. — Comparison
of coelom of Peripatus and of vertebrate. — Pronephric organs compared to
coxal glands.— Orig-in of vertebrate body-.cavity (metacoele). — Segmental
duct. — Summary of formation of excretory organs.— Origin of somatic
trunk-musculature. — Atrial cavity of Ampliioxus. — Pleural folds. — Ventral
growth of pleural folds and somatic musculature. — Pleural folds of Cepha-
laspidae and of Trilobita. — Significance of the ductless glands. — Alteration
in structure of excretory org-ans which have lost their duct in vertebrates
and in invertebrates. — Formation of lymphatic glands. — Segmental coxal
glands of arthropods and of vertebrates. — Origin of adrenals, pituitary
body, thymus, tonsils, thyroid, and other ductless glands. — Summary.

The consideration of the auditory nerve and the auditory apparatus
terminates the comparison between the cranial nerves of the verte-
brate and the prosomatic and mesosomatic nerves of the arthropod,
and leaves us now free to pass on to the consideration of the verte-
brate spinal nerves and the organs they supply. Before doing so, it
is advisable to pass in review the conclusions already attained.

Starting with the working hypothesis that the central nervous
system of the vertebrate has arisen from the central nervous system
of the arthropod, but has involved and enclosed the alimentary canal
of the latter in the process, so that there has been no reversal of
surfaces in the derivation of the one form from the other, we have been
enabled to compare closely all the organs of the head-region in the
two groups of animals, and in no single case have we been compelled
to make any startling or improbable assumptions. The simple
following out of this clue has led in every case in the most natural

2 c

386 THE ORIGIN OF VERTEBRATES

manner to the interpretation of all the organs in the head-region of
the vertebrate from the corresponding organs of the arthropod.

That it is possible to bring together all the striking resemblances
between organs in the two classes of animals, such as I have done in
preceding chapters, has been ascribed to a perverted ingenuity on my
part — a suggestion which is flattering to my imaginative powers, but
has no foundation of fact. There has been absolutely no ingenuity
on my part; all I have done is to compare organs and their nerve-
supply, as they actually exist in the two groups of animals, on the
supposition that there has been no turning over on to the back, no
reversal of dorsal and ventral surfaces. The comparison is there for
all to read ; it is all so simple, so self-evident that, given the one
clue, the only ingenuity required is on the part of those who fail
to see it.

The great distinction that has arisen between the two head-regions
is the disappearance of appendages as such, never, however, of
important organs on those appendages. If the olfactory organs of
the one group were originally situated on antennules, the olfactory
organs still remain, although the antennules as such have disap-
peared. The coxal excretory organs at the base of the endognaths
remain and become the pituitary body. A special sense-organ, such
as the fiabellum of Limulus or the pecten of scorpion, remains and
gives rise to the auditory organ. A special glandular organ, the
uterus in the base of the operculum, remains, and gives rise to the
thyroid gland. The branchia? and sense-organs on the mesosomatic
appendages remain, and even the very muscles to a large extent.
As will be seen later, the excretory organs at the base of the
metasomatic appendages remain. It is merely the appendage as
such which vanishes either by dwindling away, or by so great an
alteration as no longer to be recognizable as an appendage.

This dwindling process was already in full swing before the
vertebrate stage ; it is only a continuation of a previous tendency, as
is seen in the dwindling of the prosomatic appendages in the Mero-
stomata and the inclusion of the branchia3 within the body of the
scorpion. Already among the Pakeostraca, swimming had largely
taken the place of crawling. The whole gradual transformation from
the arthropod to the vertebrate is associated with a transformation
from a crawling to a swimming animal — with the concomitant loss
of locomotor appendages as such, and the alteration of the shape of

THE REGION OF THE SPINAL CORD 387

the animal into the lithe fish-like form. The consideration of the
manner in which this latter change was brought about, takes us
out of the cranial into the spinal region.

If we take Limulus as the only living type of the Palreostraca,
we are struck with the fact that the animal consists to all intents
and purposes of prosomatic and mesosomatic regions only ; the meta-
soma consisting of the segments posterior to the mesosoma is very
insignificant, so that the large mass of the animal consists of what
has become the head-region in the vertebrate ; the spinal region,
which has become in the higher vertebrates by far the largest region
of the body, can hardly be said to exist in such an animal as Limulus.
As to the Eurypterids and others, similar remarks may be made,
though not to the same extent, for in them a distinct raetasoma does
exist.

In this book I have considered up to the present the cranial
region as a system of segments, and shown how such segments are
comparable, one by one, with the corresponding segments in the
prosoma and mesosoma of the presumed arthropod ancestor.

In the spinal region such direct comparison is not possible, as is
evident on the face of it ; for even among vertebrates themselves the
spinal segments are not comparable one by one, so great is the varia-
tion, so unsettled is the number of segments in this region. This
meristic variation, as Bateson calls it, is the great distinctive character
of the spinal region, which distinguishes it from the cranial region
with its fixed number of nerves, and its substantive rather than
meristic variation. At the borderland, between the two regions, we
see how the one type merges into the other; how difficult it is
to fix the segmental position of the spino-occipital nerves ; how much
more variable in number are the segments supplied by the vagus
nerves than those anterior to them.

This meristic variation is a sign of instability, of want of fixedness
in the type, and is evidence, as already pointed out, that the spinal
region is newer than the cranial. This instability in the number of
spinal segments does not necessarily imply a variability in the
number of segments of the metasoma of the invertebrate ancestor;
it may simply be an expression of adaptability in the vertebrate
phylum itself, according to the recmirements necessitated by the con-
version of a crawling into a swimming animal, and the subsequent
conversion of the swimming into a terrestrial or flying animal.

388 THE ORIGIN OF VERTEBRATES

However many may have been the original number of segments
belonging to the spinal region, one thing is certain — the segmental
character of this region is remarkably clearly shown, not only by the
presence of the segmental spinal nerves, but also by the marked
segmentation of the mesoblastic structures. The question, therefore,
that requires elucidation above all others is the origin of the spinal
mesoblastic segments, i.e. of the ccelomic cavities of the trunk-region,
and the structures derived from their walls.

Proceeding on the same lines as in the case of the cranial
segments, it is necessary in the first instance to inquire of the verte-
brate itself as to the scope of the problem in this region. In addition
to the variability in the number of segments so characteristic of the
spinal region, the complete absence in each spinal segment of a
lateral root affords another marked difference between the two
regions. Here, except, of course, at the junction of the spinal and
cranial regions, each segmental nerve arises from two roots only,
dorsal and ventral, and these roots are separately sensory and motor,
and not mixed in function as was the lateral root of each cranial
segment. Now, these lateral roots were originally the nerves sup-
plying the prosomatic and mesosomatic appendages with motor as
well as sensory fibres. The absence, therefore, of lateral roots in the
spinal region implies that in the vertebrate none of the musculature
belonging to the metasomatic appendages has remained. Conse-
quently, as far as muscles are concerned, the clue to the origin of
the spinal segments must be sought for in the segmentation of the
body-muscles.

Here, in contradistinction to the cranial region, the segmentation
is most marked, for the somatic spinal musculature of all vertebrates
can be traced back to a simple sheet of longitudinal ventral and
dorsal muscles, such as are seen in all fishes. This sheet is split
into segments or myotomes by transverse connective tissue septa or
myo-commata ; each myotome corresponding to one spinal segment.

In addition to the evidence of segmentation afforded by the body-
musculature in all the higher vertebrates, similar evidence is given
by the segmental arrangement of parts of the supporting tissue to
form vertebrae. Such segments have received the name of sclerotomes,
and each sclerotome corresponds to one spinal segment.

Yet another marked peculiarity of this region is the segmental
arrangement of the excretory organs. Just as our body-musculature

THE REGION OF THE SPINAL CORD 389

has arisen from the uniformly segmented simple longitudinal muscu-
lature of the lowest fish, so, as we pass down the vertebrate phylum,
we find more and more of a uniform segmental arrangement in the
excretory organs.

The origin of all these three separate segmentations may, in
accordance with the phraseology of the day, be included in the
one term — the origin of the spinal mesoblastic segments — i.e. of the
ccelomic cavities of the trunk-region and the structures derived from
their walls.

The Origin of the Segmental Excretory Organs.

Of these three clues to the past history of the spinal region, the
segmentation manifested by the presence of vertebrae is the least
important, for in Ammoccetes there is no sign of vertebras, and their
indications only appear at transformation. Especially interesting is
the segmentation due to the excretory organs, for the evidence dis-
tinctly shows that such excretory organs have steadily shifted more
and more posteriorly during the evolution of the vertebrate.

In Limulus the excretory organs are in the prosomatic region —
the coxal glands ; these become in the vertebrate the pituitary body.

In Amphioxus the excretory organs are in the mesosomatic region,
segmentally arranged with the gills.

In vertebrates the excretory organs are in the metasomatic region
posterior to the gills, and are segmentally arranged in this region.
Their investigation has demonstrated the existence of three distinct
stages in these organs : 1. A series of segmental excretory organs in
segments immediately following the branchial segments. This is
the oldest of the three sets, and to these organs the name of the pro-
nephros is given. 2. A second series which extends more posteriorly
than the first, overlaps them to an extent which is not yet settled,
and takes their place ; to them is given the name of the mesone-
pliros. 3. A third series continuous with the mesonephric is situated
in segments still more posterior, supplants the mesonephros and
forms the kidneys of all the higher vertebrates. This forms the
metampliros.

These three sets of excretory organs are not exactly alike in their
origin, in that the pronephric tubules are formed from a different
portion of the ccelomic walls to that from which the meso- and

390 THE ORIGIN OF VERTEBRATES

metanephric tubules are formed, and the former alone gives origin
to a duct, which forms the basis for the generative and urinary
ducts, and is called the segmental duct. The mesonephric tubules,
called also the Wolffian body, open into this duct.

In order to make the embryology of these excretory organs quite
clear, I will make use of van Wijhe's phraseology and also of his
illustrations. He terms the whole ccelomic cavity the proecelom,
which is divisible into a ventral unsegmented part, the body-cavity
or metaccelom, and a dorsal segmented part, the somite. This latter
part again is divided into a dorsal part — the cpimere — and a part
connecting the dorsal part with the body-cavity, to which therefore
he gives the name of mesomere.

The cavity of the epimere disappears, and its walls form the muscle
and cutis plates of the body. The part which forms the muscles is
known as the myotome, which separates off from the mesomere, leaving
the latter as a blind sac — the mesocoelom — communicating by a narrow
passage with the body cavity or metaccelom. At the same time, from
the mesomere is formed the sclerotome, which gives rise to the skeletal
tissues of the vertebra?, etc., so that van Wijhe's epimere and mesomere
together correspond to the original term, pro to vertebra, or somite of
Balfour ; and when the myotome and sclerotome have separated
off, there is still left the intermediate cell-mass of Balfour and
Sedgwick, i.e. the sac-like mesoccele of van Wijhe, the walls of which
give origin to the mesonephrotome or mesonephros. Further, accord-
ing to van Wijhe, the dorsal part of the unsegmented metaccelom is
itself segmented, but not, as in the case of the mesoccele, with respect
to both splanchnopleuric and somatopleuric walls. The segmentation
is manifest only on the somatopleuric side, and consists of a distinct
series of hollow somatopleuric outgrowths, called by him hypomeres,
which give rise to the pronephros and the segmental duct.

Van Wijhe considers that the whole metaccelom was originally
segmented, because in the lower vertebrates the segmentation reaches
further ventral-wards, so that in Selachia the body-cavity is almost
truly segmental. Also in the gill-region of Amphioxus the cavities
which are homologous with the body-cavity arise segmentally.

As is well known, Balfour and Semper were led, from their
embryological researches, to compare the nephric organs of vertebrates
with those of annelids, and, indeed, the nature of the vertebrate
segmental excretory organs has always been the fact which has kept

THE REGION OF THE SPINAL CORD

»9i

Fig. 156. — Diagrams to illustrate the Development of the Vertebrate

Ccelom. (After van Wijhe.)

JV., central nervous system; JVc, notochord ; Ao., aorta; Mg., midgut. A, My.,
myoccele ; Mes., mesoccele ; Met., metaceele ; Hyp., hypomere (pronephric). B
and C, My., myotome; Mes., mesoncphros ; S.d., segmental duct (pronephric) ;
Met., body cavity.

392 THE O RIG IX OF VERTEBRATES

alive the belief in the origin of vertebrates from a segmented annelid.
These segmental organs thus compared were the mesonephric tubules,
and doubts arose, especially in the mind of Gegenbaur, as to the
validity of such a comparison, because the mesonephric tubules did
not open to the exterior, but into a duct — the segmental duct — which
was an unseginented structure opening into the cloaca ; also because
the segmental duct, which was the excretory duct of the pronephros,
was formed first, and the mesonephric tubules only opened into
it after it was fully formed. Further, the pronephros was said to
arise from an outbulging of the somatopleuric mesoblast, which
extended over a limited number of metameres, and was not segmental,
but continuous. Gegenbaur and others therefore argued that the
original prevertebrate excretory organ was the pronephros and its duct,
not the mesonephros, from which they concluded that the vertebrate
must have been derived from an unseginented type of animal, and
not from the segmented annelid type.

Such a view, however, has no further reason for acceptance, as
it was based on wrong premises, for Euckert has shown that the
pronephros does arise as a series of segmental nephric tubules, and
is not unsegmented. He also has pointed out that in Torpedo the
anterior part of the pronephric duct shows indications of being seg-
mented, a statement fully borne out by the researches of Maas on
Myxine, who gives the clearest evidence that in this animal the
anterior part of the pronephric duct is formed by the fusion of a
series of separate ducts, each of which in all probability once
opened out separately to the exterior.

Euckert therefore concludes that Balfour and Semper were right
in deriving the segmental organs of vertebrates from those of annelids,
but that the annelid organs are represented in the vertebrate, not by
the mesonephric tubules, but by the pronephric tubules and their
ducts, which originally opened separately to the exterior. By the
fusion of such tubules the anterior part of the segmental duct was
formed, while its posterior part either arose by a later ccenogenetic
lengthening, or is the only remnant of a series of pronephric tubules
which originally extended the whole length of the body, as suggested
also by Maas and Boveri. Euckert therefore supposed that the
mesonephric tubules were a secondary set of nephric organs, which
were not necessarily directly derived from the annelid nephric
organs.

THE REGION OF THE SPINAL CORD 393

At present, then, Biickert's view is the one most generally ac-
cepted— the original annelid nephric organs are represented by the
pronephric tubules and the pronephric duct, not by the mesonephric
tubules, which are a later formation. This latter statement would
hold good if the mesonephric tubules were found entirely in seg-
ments posterior to those containing the pronephric tubules ; such,
however, is said not to be the case, for the two sets of organs are
said to overlap in some cases ; even when they exist in the same
segments, the former are said always to be formed from a more
dorsal part of the ccelom than the pronephros, always to be a later
formation, and never to give any indication of communicating with
the exterior except by way of the pronephric duct.

The recent observations of Brauer on the excretory organs of the
Gymnophiona throw great doubt on the existence of mesonephric and
pronephric tubules in the same segment. He criticizes the observa-
tions on which such statements are based, and concludes that, as in
Hypogeophis, the nephrotome which is cut off after the separation of
the sclero-myotome gives origin to the pronephros in the more anterior
regions, just as it gives origin to the mesonephros in the more
posterior regions. In fact, the observations of van Wijhe and others
do not in reality show that two excretory organs may be formed
in one segment, the one mesonephric from the remains of the meso-
mere and the other pronephric from the hypomere, but rather that
in such cases there is only one organ — the pronephros — part of which
is formed from the mesomere and part from the hypomere. Brauer
goes further than this, and doubts the validity of any distinction
between pronephros and mesonephros, on the ground of the former
arising from a more ventral part of the proccelom than the latter ;
for, as he says, it is only possible to speak of one part of the somite
as being more ventral than another part when both parts are in the
same segment ; so that if pronephric and mesonephric organs are
never in the same segment, we cannot say with certainty that the
former arises more ventrally than the latter.

These observations of Brauer strongly confirm Sedgwick's original
statement that the pronephric and mesonephric organs are honio-
dynamous organs, in that they are both derived from the original
serially situated nephric organs, the differences between them being
of a subordinate nature and not sufficient to force us to believe that
the mesonephros is an organ of quite different origin to the

394 THE ORIGIN OF VERTEBRATES

pronephros. So, also, Price, from his investigations of the excretory-
organs of Bdellostoma, considers that in this animal both pro-
nephros and mesonephros are derived from a common embryonic
kidney, to which he gives the name Jwlonejrfiros.

Brauer also is among those who conclude that the vertebrate
excretory organs were derived from those of annelids ; he thinks that
the original ancestor possessed a series of similar organs over the
whole pronephric and mesonephric regions, and that the anterior
pronephric organs, which alone form the segmental duct, became
modified for a larval existence — that their peculiarities were adaptive
rather than ancestral. This last view seems to me very far-fetched,
without any sufficient basis for its acceptance. According to the
much more probable and reasonable view, the pronephros represents
the oldest and original excretory organs, while the mesonephros
is a later formation. Brauer's evidence seems to me to signify that
the pronephros, mesonephros, and metanephros are all serially homo-
logous, and that the pronephros bears much the same relation to
the mesonephros that the mesonephros does to the metanephros.
The great distinction of the pronephros is that it, and it alone,
forms the segmental duct.

We may sum up the conclusions at which we have now arrived
as follows : —

1. The pronephric tubules and the pronephric duct are the oldest
part of the excretory system, and are distinctly in evidence for a
few segments only in the most anterior part of the trunk-region
immediately following the branchial region. They differ also from
the mesonephric tubules by not being so clearly segmental with the
myotomes.

2. The mesonephric tubules belong to segments posterior to those
of the pronephros, are strictly segmental with the myotomes, and
open into the pronephric duct.

3. All observers are agreed that the two sets of excretory organs
resemble each other in very many respects, as though they arose
from the same series of primitive organs, and, according to Sedgwick
and Brauer, no distinction of any importance does exist between
the two sets of organs. Other observers, however, consider that the
pronephric organs, in part at all events, arise from a part of the
nephrocoele more ventral than that which gives origin to the mesone-
phric organs, and that this difference in position of origin, combined

THE REGION OF THE SPINAL CORD 395

with the formation of the segmental duct, does constitute a true
morphological distinction between the two sets of organs.

4. All the recent observers are in agreement that the vertebrate
excretory organs strongly indicate a derivation from, the segmental
organs of annelids.

The very strongest support has been given to this last conclusion
by the recent discoveries of Boveri and Goodrich upon the excretory
organs of Amphioxus. According to Boveri, the nephric tubules of
Amphioxus open into the dorsal ccelom by one or more funnels.
Around each funnel are situated groups of peculiar cells, called by
him ' Fadenzellen,' each of which sends a long process across the
opening of the funnel. Goodrich has examined these ' Fadenzellen,'
and found that they are typical pipe- cells, or solenocytes, such as he
has described in the nephridial organs of various members of the
aunelid group Polychseta. Also, just as in the Polyclnrta, the ciliated
nephric tubule has no internal funnel-shaped opening into the ccelom,
but terminates in these groups of solenocytes. " Each solenocyte
consists of a cell-body and nucleus situated at the distal free
extremity of a delicate tube ; the proximal end of the tube pierces
the wall of the nephridial canal and opens into its lumen. A single
long flagellum arising from the cells works in the tube and projects
into the canal."

The exceedingly close resemblance between the organs of
Amphioxus and those of Phyllodoce, as given in his paper, is most
striking, and, as he says, leads to the conclusion that the excretory
organs of Amphioxus are essentially identical with the nephridia of
certain polychsete worms.

It is to me most interesting to find that the very group of
annelids, the Polychteta, which possess solenocytes so remarkably
resembling those of the excretory organs of Amphioxus, are the
highest and most developed of all the Annelida. I have argued
throughout that the law of evolution consists in the origination of
successive forms from the dominant group then alive, dominance
signifying the highest type of brain-power achieved up to that time.
The highest type among Annelida is found in the Chretopoda ; from
them, therefore, the original arthropod type must have sprung.
This original group of Arthropoda gave rise to the two groups of
Crustacea and Arachnida, in my opinion also to the Vertebrata,
and, as already mentioned, it is convenient to give it a generalized

396 THE ORIGIN OF VERTEBRATES

name, the Protostraca, from which subsequently the Palasostraca

arose.

The similarity between the excretory organs of Amphioxus and
those of Phyllodoce suggests that the protostracan ancestor of the
vertebrates arose from the highest group of the Chaetopoda — the
Polychseta. The evidence which I have already given points, how-
ever, strongly to the conclusion that the vertebrate did not arise from
members of the Protostraca near to the polychtete stock, but rather
from members in which the arthropod characters had already become
well developed— members, therefore, which were nearer the Trilobita
than the Polychreta. Such early arthropods would very probably
have retained in part excretory organs of the same character as those
found in the original polychrete stock, and thus account for the
presence of solenocytes in the excretory organs of Amphioxus.

In connection with such a possibility, I should like to draw
attention to the observations of Glaus and Spangenberg on the
excretory organs of Branchipus— that primitive phyllopod, which is
recognized as the nearest approach to the trilobites at present living.
According to Glaus, an excretory apparatus exists in the neighbour-
hood of each nerve-ganglion, and Spangenberg finds a perfectly
similar organ in the basal segment of each appendage — a system,
therefore, of excretory organs as segmentally arranged as those of
Peripatus. Claus considers that although these organs formed an
excretory system, it is not possible to compare them with the
annelid segmental organs, because he thought the cells in question
arose from ectoderm. Now, the striking point in the description of
the excretory cells in these organs, as described both by Claus and
Spangenberg, is that they closely resemble the pipe-cells or sole-
nocytes of Goodrich ; each cell possesses a long tube-like projection,
which opens on the surface. They appear distinctly to belong to the
category of flame-cells, and resemble solenocytes more than anything
else. According to Goodrich, the solenocy te is probably an ectodermal
cell, so that even if it prove to be the case, as Claus thought, that
these pipe-cells of Branchipus are ectodermal, they would still claim
to be derived from the segmental organs of annelids, especially of the
Polychaeta, being, to use Goodrich's nomenclature, true nephridial
organs, as opposed to ccelomostomes.

These observations of Claus and Spangenberg suggest not only
that the primitive arthropod of the trilobite type possessed segmental

THE REGION OF THE SPINAL CORD 397

organs in every segment directly derived from those of a polyctuete
ancestor, but also that such organs were partly somatic and partly
appendicular in position. Such a suggestion is in strict accord with
the observations of Sedgwick on the excretory organs of the most primi-
tive arthropod known, viz. Peripatus, where also the excretory organs,
which are true segmental organs, are partly somatic and partly
appendicular. Further, the excretory organs of the Scorpion and
Limulus group are again partly somatic and partly appendicular,
receiving the name of coxal glands, because there is a ventral projec-
tion of the gland into the coxa of the corresponding appendage.

Judging from all the evidence available, it is probable that when
the arthropod stock arose from the annelids, simultaneously with the
formation of appendages, the segmental somatic nephric organs of
the latter extended ventrally into the appendage, and thus formed
a segmental set of excretory organs, which were partly somatic,
partly appendicular in position, and might therefore be called coxal
glands.

As already stated, all investigators of the origin of the vertebrate
excretory organs are unanimous in considering them to be derived
from segmental organs of the annelid type. I naturally agree with
them, but, in accordance with my theory, would substitute the words
" primitive arthropod " for the word " annelid," for all the evi-
dence I have accumulated in the preceding chapters points directly
to that conclusion. Further, the most primitive of the three sets
of vertebrate segmental organs — the pronephros, mesonephros, and
metanephros — is undoubtedly the pronephros ; consequently the
pronephric tubules are those which I consider to be more directly
derived from the coxal glands of the primitive arthropod ancestor.
Such a derivation appears to me to afford an explanation of the diffi-
culties connected with the origin of the pronephros and mesonephros
respectively, which is more satisfactory than that given by the direct
derivation from the annelid.

The only living animal which we know of as at all approaching
the most primitive arthropod type is, as pointed out by Korschelt and
Heider, Peripatus ; and Peripatus, as is well known, possesses a true
ccelom and true ccelomic excretory organs in all the segments of the
body. Sedgwick shows that at first a true ccelom, as typical as that
of the annelids, is formed in each segment of the body, and that then
this ccelom (which represents in the vertebrate van Wijhe's pro-ccelom)

^9$ THE ORIGIN OF VERTEBRATES

splits into a dorsal and a ventral part. In the anterior segments of
the body the dorsal part disappears (presumably its walls give origin
to the mesoblast from which the dorsal body-muscles arise), while
the ventral part remains and forms a nephroccele, giving origin to. the
excretory organs of the adult. According to von Kennel, the cavity
becomes divided into three spaces, which for a time are in com-
munication— a lateral (I.), a median (II.), and a dorso-median (III.).
The dorso-median portion becomes partitioned off, and this, as well
as the greater part of the lateral portion, which lies principally in
the foot, is used up in providing elements for the formation of the
body- and appendage-muscles respectively and the connective tissue.

In Fig. 157 I reproduce von Kennel's diagram of a section across
a Peripatus embryo, in which I. represents the lateral appendicular
part of the ccelom, II. the ventral somatic part, and III. the dorsal
part which separates off from the ventral and lateral parts, and, as
its walls give origin largely to the body-muscles, may be called
the myoccele. The muscles of the appendages are formed from
the ventral part of the original procoelom, just as I have argued
is the case with the muscles of the splanchnic segmentation in
vertebrates.

Sedgwick states that the ventral part of the ccelom extends
into the base of each appendage, and there forms the end-sac of
each nephric tubule, into which the nephric funnel opens, thus
forming a coxal gland ; this end-sac or vesicle in the appendage
is called by him the internal vesicle (i.v.), because later another
vesicle is formed from the ventral ccelom in the body itself, close
against the nerve-cord on each side, which he calls the external
vesicle (e.v.). (Cf. Fig. 158, taken from Sedgwick.) This second
vesicle is, according to him, formed later in the development from
the nephric tubule of the internal vesicle, so that it discharges
its contents to the exterior by the same opening as the original
tubule. Of course, as he points out, the whole system of internal
and external vesicles and nephric tubules are all simply derivatives
of the original ventral part of the ccelom or nephroccele.

Here, then, in Peripatus, and presumably, therefore, in members
of the Protostraca, we see that the original segmental organs of the
annelid have become a series of nephric organs, which extended into
the base of the appendages, and may therefore be called coxal glands ;
also it is clear, from Sedgwick's description, that if the appendages

THE REG 10 X OF THE STIXAL CORD

399

disappeared, the nephric organs would still remain, not as coxal
glands, but as purely somatic excretory glands. They would still be
homologous with the annelid segmental organs, or with the coxal
glands, but would arise in toto from a part of the ventral ccelom or
nephroccele, more dorsal than the former appendicular part, because
the appendages and their enclosed ccelom are always situated ventrally
to the body. Again, according to Sedgwick, the nephric tubules are

App

Fig. 157. — Transvkrse Section of Peripatus Embryo. (After von Kennel.)

Al., alimentary canal; N., nerve-cord; App., appendage; I, II, III, the three
divisions (lateral, median, and dorso-median) of the ccelom.

e.v
i v

App

Fig. 158. — Section op Peripatus. (After Sedgwick.)

Al., alimentary canal; N., nerve-cord; App., appendage; i.v., internal, and e.v.,
external vesicles of the segmented excretory tubule (coxal gland).

connected with two ccelomic vesicles, the one in the appendage the
internal vesicle, and the other, the so-called bladder, or the external
vesicle, in the body itself, close against the nerve-cord. Sedgwick
appears to consider that either of these vesicles may form the end-
sac of a nephric tubule, for he discusses the question whether the
single vesicle, which in each case gives origin to the nephridia of the
first three legs, corresponds to the internal or external vesicle. He

4-00 THE O RIG IX OF VERTEBRATES

decides, it is true, in favour of the internal vesicle, and therefore
considers the excretory organ to be appendicular, i.e. a coxal gland, in
these segments as well as in those more posterior. Still, the very
discussion shows that in his opinion, at all events, the external
vesicle might represent the end-sac of the tubule, in the absence of
the internal or appendicular vesicle.

Such an arrangement as Sedgwick describes in Peripatus is the
very condition required to give rise to the pronephric and meso-
nephric tubules, as deduced by me from the consideration of the
vertebrate, and harmonizes and clears up the controversy about the
mesonephros and pronephros in the most satisfactory manner. Both
pronephros and mesonephros are seen to be derivatives of the original
annelid segmental organs, not directly from an annelid, but by way
of an arthropodan ancestor; the difference between the two is
simply that the pronephric organs were coxal glands, and indi-
cate, therefore, the presence of the original metasomatic appendages,
while the mesonephric organs were homologous organs, formed in
segments of later origin which had lost their appendages. For this
reason the pronephros is said to be formed, in part at least, from
a portion of the cceloin situated more ventrally than the purely
somatic part which gives rise to the mesonephros. For this reason
Sedgwick, Brauer, etc., can say that the mesonephros is strictly homo-
dynamous with the pronephros; while equally Kiickert, Semon, and van
Wijhe can say it is not homodynamous, in so far that the two organs
are not derived strictly from absolutely homologous parts of the coelom.
For this reason Semon can speak of the mesonephros as a dorsal
derivative of the pronephros, just as Sedgwick says that the external
or somatic vesicle of Peripatus is a derivative of the appendicular
nephric organ. For this reason the pronephros, or rather a part of it,
is always derived from the somatopleuric layer, for, as is clear from
Miss Sheldon's drawing, the part of the ccelom in Peripatus which
dips into the appendage is derived from the somatopleuric layer
alone.

Such a ccelom as that of Peripatus, Fig. 157, would represent the
origin of the vertebrate ccelom, and would therefore represent the
proccelom of van Wijhe. In strict accordance with this, we see that it
separates into a dorsal part, the walls of which give origin to the
somatic muscles, or at all events to the great longitudinal dorsal
muscles of the animal, and a ventral part, which forms a nephroccele,

THE REGION OF THE SPINAL CORD 401

dips into the appendage, and gives origin to the muscles of the
appendage. In the vertebrate, after the somatic dorsal part or
myoccele has separated off, a ventral part is left, which forms a
nephroccele in the trunk-region, and gives origin to the splanchnic
striated muscles in the cranial region, i.e. to the muscles which,
according to my theory, were once appendicular muscles. This
ventral nephroccelic part is divisible in the trunk into a segmented
part, which forms the excretory organs proper, and an unsegmented
part, the metaccele or true body-cavity of the vertebrate.

This comparison of the procoelom of the vertebrate and arthropod
signifies that the vertebrate metaccele was directly derived by ventral
downgrowth from the arthropod nephroccele, so that if, as I suppose,
the vertebrate nervous system represents the conjoined nervous
system and alimentary canal of the arthropod, then the vertebrate
metaccele, or body-cavity, must have been originally confined to the
region on each side of the central nervous system, and from this
position have spread ventrally, to enclose ultimately the new-formed
vertebrate gut. This means that the body-cavity (metaccele) of the
vertebrate is not the same as the body-cavity of the annelid, but
corresponds to a ventral extension of the nephroccele, or ventral part
of such body-cavity.

Such a phylogenetic history is most probable, because it explains
most naturally and simply the facts of the development of the verte-
brate body-cavity ; for the mesoblast always originates in the neigh-
bourhood of the notochord and central nervous system, and the lumen
of the body- cavity always appears first in that region, and then
extends laterally and ventrally on each side until it reaches the most
ventral surface of the embryo, thus forming a ventral mesentery,
which ultimately disappears, and the body-cavity surrounds the gut,
except for the dorsal mesentery. Thus Shipley, in his description of
the formation of the mesoblastic plates which line the body- cavity in
Ammoccetes, describes them as commencing in two bands of meso-
blast situated on each side, close against the commencing nervous
system: —

" These two bands are separated dorsally by the juxtaposition of
the dorsal wall of the mesenteron and the epiblast, and ventrally by
the hypoblastic yolk- cells which are in contact with the epiblast
over two-thirds of the embryo. Subsequently, but at a much later
date, the mesoblast is completed ventrally by the downgrowth on

2 D

402 THE ORIGIN OF VERTEBRATES

each side of these mesohlastic plates. The subsequent downward
growth is brought about by the cells proliferating along the free
ventral edge of the mesoblast, these cells then growing ventralwards,
pushing their way between the yoke-cells and epiblast."-

The derivation of the vertebrate pronephric segmental organs
from the metasomatic coxal glands of a primitive arthropod would
mean, if the segmental organs of Peripatus be taken as the type,
that such glands opened to the exterior on every segment, either
at the base of the appendage or on the appendage itself. It is
taken for granted by most observers that the pronephric segmental
organs once opened to the exterior on each segment, and then,
from some cause or other, ceased to do so, and the separate ducts,
by a process of fusion, came to form a single segmental duct, which
opened into the cloaca. Many observers have been led to the con-
clusion that the pronephric duct is epiblastic in origin, although
from its position in the adult, it appears far removed from all
epiblastic formations. However, at no time in the developmental
history is there any clear evidence of actual fusion of any part of the
pronephric organ with the epidermis, and the latest observer, Brauer,
is strongly of opinion that there is never sufficiently close contact
with the epidermis to warrant the statement that the epiblastic cells
take part in the formation of the duct. All that can be said is, that
the formation of the duct takes place at a time when the pronephric
diverticulum is in close propinquity to the epidermis, before the
ventral downgrowth of the myotome has taken place.

The formation of the anterior portion of the pronephric duct is,
according to Maas in Myxine, and Wheeler in Petromyzon, undoubtedly
brought about by the fusion of a number of pronephric tubules, which,
according to Maas, are clearly seen in the youngest specimens as
separate segmental tubes; each of these tubules is supplied by a
capillary network from a segmental branch of the aorta, as in the
tubules of Amphioxus according to Boveri, and does not possess a
glomerulus.

The posterior part of the duct into which the mesonephric tubules
enter possesses also a capillary network, which Maas considers to
represent the original capillary network of a series of pronephric
tubules, the only remnant of which is the duct into which the
mesonephric tubules open. He therefore argues that the pronephric
duct indicates a series of pronephric tubules, which originally extended

THE REGION OF THE SPINAL CORD 40

along the whole length of the body, and were supplanted by the
mesonephric tubules, which also belonged to the same segments.

I also think that the paired appendages which have left the pro-
nephric tubules as signs of their past existence, existed originally, in
the invertebrate stage, on every segment of the body. But I do not
consider that such a statement is at all equivalent to saying that such
pairs of tubules must have existed upon every one of the segments
existing at the present clay ; for it seems to me that Eiickert is much
more likely to be right when he says that in Selachians the duct
clearly does grow back, and is not formed throughout in situ ,• so that
he gives a double explanation of the formation of the duct — a palin-
genetic anterior part formed by the fusion of the extremities of the
original excretory tubules, to which a posterior ccenogenetic lengthen-
ing has been added.

It does not seem to me at all necessary that the immediate inver-
tebrate ancestor of the vertebrate should have possessed excretory
organs which opened out separately to the exterior on each segment ;
a fusion may already have taken place in the invertebrate stage, and
so a single duct have been acquired for a number of organs. Such a
suggestion has been made by Eiickert, because of the fact discovered
by Cunningham and E. Meyer, that the segmental organs of Lanice
conchilega are on each side connected together by a single strong
longitudinal canal. I would, however, go further than this and say,
that even although the nephric organs of the polychsete ancestor
opened out on every segment, and although the primitive arthropodan
ancestor derived from such polychaite possessed coxal glands which
opened out either on to or at the base of each appendage, similarly to
those of Peripatus, yet the immediate arthropodan ancestor, with its
palseostracan affinities, may already have possessed metasomatic coxal
glands, all of which opened into a single duct, with a single opening
to the exterior.

Judging from Limulus, such was very probably the case, for
Patten and Hazen have shown (1) that the coxal glands of Limulus
are segmental organs belonging to the prosomatic segments ; (2) that
the organs belonging to the cheliceral and ectognathal segments
are not developed ; (3) that the four glands belonging to the endo-
gnaths become connected together by a stolon, which communicates
with a single nephric duct, opening to the exterior on the basal
segment of the 5th prosomatic appendage (the last endognath). At

404 THE ORIGIN OF VERTEBRATES

no time is there any evidence of any separate openings or any fusion
with the ectoderm, such as might indicate separate openings of these
prosomatic coxal segmental organs. Thus we see that in Limulus,
which is presumably much nearer the annelid condition than the
vertebrate, all evidence of separate nephric ducts opening to the
exterior on each prosomatic segment has entirely disappeared, just as
is the case in the metasomatic coxal glands (i.e. the pronephros) of
the vertebrate. What is seen in the prosomatic region of Limulus,
and doubtless also of the Eurypterids, may very probably have
occurred in the metasomatic region of the immediate invertebrate
ancestors of the vertebrate, and so account for the single pro-
nephric duct belonging to a number of pronephric organs.

The interpretation of these various embryological investigations
may be summed up as follows : —

1. The ancestor of the vertebrates possessed a pair of appendages
on each segment ; into the base of each of these appendages the
segmental excretory organ sent a diverticulum, thus forming a coxal
gland.

2. Such coxal glands, even in the invertebrate stage, may have
discharged into a common duct which opened to the exterior most
posteriorly.

3. Then, from some cause, the appendages were rendered useless,
and dwindled away, leaving only the pronephric organs to indicate
their former presence. At the end of this stage the animal possessed
vertebrate characteristics.

4. For the purpose of increasing mobility, of forming an efficient
swimming instead of a crawling animal, the body-segments increased
in number, always, as is invariably the case, by the formation of new
ones between those already formed and the cloacal region, and so of
necessity caused an elongation of the pronephric duct. Into this there
now opened the ducts of the segmental organs formed by recapitula-
tion, those, therefore, belonging to the body-segments — mesonephric —
having nothing to do with appendages, for the latter had already
ceased to exist functionally, and would not, therefore, be repeated with
each meristic repetition.

This, so to speak, passive lengthening of the pronephric duct in
consequence of the lengthening of the early vertebrate body by the
addition of metameres, each of which contained only mesonephric
and no pronephric tubules, is, to my mind, an example of a principle

THE REGION OF THE SPINAL CORD 405

which has played an important part in the formation of the verte-
brate, viz. that the meristic variation by which the spinal region of
even the lowest of existing vertebrates has been formed, has largely
taken place in the vertebrate phylum itself, and that such changes
must be eliminated before we can picture to ourselves the pre-verte-
brate condition. As an example, I may mention the remarkable
repetition of similar segments pictured by Bashford Dean in Bdello-
stoma. Such repetition leads to passive lengthening of such parts
as are already formed but are not meristically repeated : such are the
notochord, the vertebrate intestine, the canal of the spinal cord, and
possibly the lateral line nerve. The fuller discussion of this point
means the discussion of the formation of the vertebrate alimentary
canal ; I will therefore leave it until I come to that part of my
subject, and only say here that the evidence seems to me to point to
the conclusion that at the time when the vertebrate was formed, the
respiratory and cloacal regions were very close together, the whole of
the metasoma being represented by the region of the pronephros
alone.

Here, as always, the evidence of Ammoccetes tends to give
definiteness to our conceptions, for Wheeler points out that up to a
length of 7 mm. the pronephros only is formed ; there is no sigu of
the more posteriorly formed mesonephros. Now we know, as pointed
out in Chapter VI., p. 228, this is the time of Kupffer's larval stage
of Ammoccetes. This is the period during which the invertebrate
stage is indicated in the ontogeny, so that, in accordance with all
that has gone before, this means that the metasoma of the inverte-
brate ancestor was confined to the region of the pronephros.

Again, take Shipley's account of the development of Petromyzon.
He says —

" The alimentary canal behind the branchial region may be
divided into three sections. Langerhans has termed these the stomach,
midgut, and hindgut, but as the most anterior of these is the narrowest
part of the whole intestine, it would, perhaps, be better to call it
oesophagus. This part of the alimentary canal lies entirely in front
of the yolk, and is, with the anterior region, which subsequently
bears the gills, raised from the rest of the egg when the head is
folded off. It is supported by a dorsal mesentery, on each side of
which lies the head-kidney (pronephros)."

Further on he says —

406 THE ORIGIN OF VERTEBRATES

"The hindgut is smaller than the midgut; its anterior limit is
marked by the termination of the spiral valve, which does not extend
into this region. The two segmental ducts open into it just where it
turns ventrally to open to the exterior by a median ventral anus.
Its lumen is from an early stage lined with cells which have lost
their yolk, and it is in wide communication with the exterior from
the first. This condition seems to be, as Scott suggests, connected
with the openings of the ducts of the pronephros, for this gland is
completed and seems capable of functioning long before any food
could find its way through the midgut, or, indeed, before the stomo-
dieum has opened."

Is there no significance in this statement of Shipley ? Even if it
be possible to find some special reason why the branchial and cloacal
parts of the gut are freed from yolk and lined with serviceable
epithelium a long time before the midgut, why should a bit of the
midgut, which Shipley calls the oesophagus, which is connected with
the region of the pronephros and not of the branchiae, differ so
markedly from the rest of the midgut ? Surely the reason is that
the branchial region of the gut, the pronephric region of the gut, and
the cloacal region of the gut, belong to a different and earlier phase
in the phylogenetic history of the Ammoccetes than does the midgut
between the pronephric and cloacal regions. This observation of
Shipley fits in with and emphasizes the view that the original animal
from which the vertebrate arose consisted of a cephalic and branchial
region, followed by a pronephric and cloacal region ; the whole inter-
mediate part of the gut, which forms the midgut, with its large lumen
and spiral valve, and belongs to the mesonephric region, being a later
formation brought about by the necessity of increasing the length of
the body.

The OrjGiN of the Somatic Tkunk-Musculature and the

FORMATION OF AX ATRIAL CAVITY.

Next comes the question, why was the pronephros not repeated
in the meristic repetition that took place during the early vertebrate
stage ? What, iu fact, caused the disappearance of the metasomatic
appendages, and the formation of the smooth body-surface of the fish ?

The embryological evidence given by van Wijhe and others of
the manner in which the original superficially situated pronephros is

THE REGION OF THE SPINAL CORD 407

removed from the surface and caused to assume the deeper position,
as seen in the later embryo, is perfectly clear and uniform in all the
vertebrate groups. The diagrams at the end of van Wijhe's paper,
which I reproduce here, illustrate the process which takes place. At
first the myotome (Fig. 159, A) is confined to the dorsal region on
each side of the spinal cord and notochord. Then (Fig. 159, B) it
separates from the rest of the somite and commences to extend ven-
trally, thus covering over the pronephros and its duct, until finally
(Fig. 159, C) it reaches the mid-ventral line on each side, and the
foundations of the great somatic body-muscles are finally laid.

In order, therefore, to understand how the obliteration of the
appendages took place, we must first find out what is the past history
of the myotomes. Why are they confined at first to the dorsal region
of the body, and extend afterwards to the ventral region, forcing by
their growth an organ that was originally external in situation to
become internal ?

In the original discussion at Cambridge, I was accused of violating
the important principle that in phylogeny we must look at the most
elementary of the animals whose ancestors we seek, and was told
that the lowest vertebrate was Amphioxus, not Ammoccetes ; that
therefore any argument as to the origin of vertebrates must proceed
from the consideration of the former and not the latter animal. My
reply was then, and is still, that I was considering the cranial region
in the first place, and that therefore it was necessary to take the
lowest vertebrate which possessed cranial nerves and sense-organs of
a distinctly vertebrate character, a criterion evidently not possessed
by Amphioxus. Such argument does not apply to the spinal region,
so that, now that I have left the cranial region and am considering
the spinal, I entirely agree with my critics that Amphioxus is likely
to afford valuable help, and ought to be taken into consideration as
well as Ammoccetes. The distinction between the value of the spinal
(including respiratory) and cranial regions of Amphioxus for drawing
phylogenetic conclusions is recognized by Boveri, who says that, in
his opinion, " Amphioxus shows simplicity and undifferentiation
rather than degeneration. If truly Amphioxus is somewhat degene-
rated, then it is so in its prehensile and masticatory apparatus> its
sense organs, and perhaps its locomotor organs, owing to its method
of living."

Hatschek describes in Amphioxus how the coelom splits into a

4o8

THE ORIGIN OF VERTEBRATES

JVc

--Mel
-Mg

Fig. 159. — Diagrams to illustrate the Development of the Vertebrate

Ccelom. (After van Wijhe.)

AT., central nervous system; Arc, notochord ; Ao., aorta; Mg., midgut. A, My.,
myoccele ; Mes., mesoccele ; Met., metacoele ; Hyp., hypomere (pronephric). B
and C, My., myotome; Mes., mesonephros ; S.cl., segmental duct (pronephric);
Met., body-cavity.

THE REGION OF THE SPINAL CORD 409

dorsal segmented portion, the protovertebra, and a ventral unseg-
mented portion, the lateral plates. He describes in the dorsal part
the formation of myotome and sclerotome, as in the Craniota.
Also, he describes how the myotome is at first confined to the dorsal
region in the neighbourhood of the spinal cord and notochord, and
subsequently extends ventrally, until, just as in Ammoccetes, the
body is enveloped in a sheet of somatic segmented muscles, the well-
known myomeres.

The conclusion to be drawn from this is inevitable. Any explana-
tion of the origin of the somatic muscles in Ammoccetes must also
be an explanation of the somatic muscles in Aniphioxus, and con-
versely ; so that if in this respect Aniphioxus is the more primitive
and simpler, then the condition in Ammoccetes must be looked upon
as derived from a more primitive condition, similar to that found in
Aniphioxus. Now, it is well know that a most important distinction
exists between Aniphioxus and Ammoccetes in the topographical
relation of the ventral portion of this muscle-sheet, for in the former
it is separated from the gut and the body-cavity by the atrial space,
while in the latter there is no such space. Fiirbringer therefore
concludes, as I have already mentioned, that this space has become
obliterated in the Craniota, but that it must be taken into considera-
tion in any attempt at formulating the nature of the ancestors of the
vertebrate.

Kowalewsky described this atrial space as formed by the ventral
downgrowth of pleural folds 011 each side of the body, which met in
the mid-ventral line and enclosed the branchial portion of the gut.
According to this explanation, the whole ventral portion of the
somatic musculature of the adult Aniphioxus belongs to the extension
of the pleural folds, the original body-musculature being confined to
the dorsal region. This is expressed roughly on the external surface
of Aniphioxus by the direction of the connective tissue septa between
the myotomes (c/. Fig. 162, B). These septa, as is well known, bend
at an angle, the apex of which points towards the head. The part
dorsal to the bend represents the part of the muscle belonging to the
original body ; the part ventral to the bend is the pleural part, and
represents the extension into the pleural folds.

Lankester and Willey have attempted to give another explanation
of the formation of the atrial cavity ; they look upon it as originating
from a ventral groove, which becomes a canal by the meeting of two

4IO THE ORIGIN OF VERTEBRATES

outgrowths from the metapleure on each side. This canal then
extends dorsal wards on each side, and so forms the atrial cavity ; the
metapleure still remains in the adult ; the somatic muscles in the
epipleure of the adult are the original body-muscles, and not exten-
sions into an epipleuric fold, for there is no such fold.

This explanation is a possible conception for the post-branchial
portion of the atrium, but is impossible for the branchial region ; for,
as Macbride points out, as must necessarily be the case, the point of
origin of the atrial wall is, in all stages of development, situated at
the end of the gill-slit. It shifts in position with the position of the
gill-slit, but there can be no backwards extension of the cavity.
Macbride therefore agrees with Kowalewsky that the atrial cavity is
formed by the simultaneous ventral extension of pleural folds, and of
the branchial part of the original pharynx. Thus, in his summing up,
he states : " In the larva practically the whole sides and dorsal
portion of the pharynx represent merely the hyper-pharyngeal groove
and the adjacent epithelium of the pharynx of the adult, the whole
of the branchial epithelium of the adult being represented by a very
narrow strip of the ventral wall of the pharynx of the larva. The
subsequent disproportionate growth of this part of the pharynx of
the larva, and of the adjacent portion of the atrial cavity, has given
the impression that the atrial cavity grew upwards and displaced
other structures, which is not the case."

Further, van Wijhe states that the atrium extends beyond the
atriopore right up to the anus, just as must have been the case if the
pleural folds originally existed along the whole length of the body.
His words are : " Allerdings hat sich das Atrium beim Ampliloxus
lanccolatus eigenthumlich ausgebildet, indem sich dasselbe durch
den ganzen Eumpf bis an den Anus, d.h. bis an die Wurzel des
Schwanzes ausdehnt."

We get, therefore, this conception of the origin of the somatic
musculature of the vertebrate. The invertebrate ancestor possessed
on each side, along the whole length of its body, a lateral fold or
pleuron which was segmented with the body, and capable of move-
ment with the body, because the dorsal longitudinal somatic muscles
extended segmentally into each segment of the pleuron. By the
ventral extension of these pleural folds, not only was the smooth
body-surface of the vertebrate attained, but also the original appen-
dages obliterated as such, leaving only as signs of their existence the

THE REGION OF THE SPINAL CORD 4 1 I

branchiae, the pronephric tubules, and the sense-organs of the lateral
line system.

Such an explanation signifies that the somatic trunk-musculature
of the vertebrate was derived from the dorsal longitudinal muscula-
ture of the body of the arthropod, and not from the ventral longitu-
dinal musculature, and that therefore in the primitive arthropod stage
the equivalent of the myotome of the vertebrate did not give origin
to the ventral longitudinal muscles of the invertebrate ancestor.
Now, as I have said, von Kennel states that in the procoelom
of Peripatus a dorsal part (III. in Fig. 157) is cut off which gives
origin to the dorsal body-musculature, while the ventral part which
remains (I. and II. in Fig. 157) gives origin in its appendicular
portion (I.) to the muscles of the appendage, and presumably in its
ventral somatic portion (II.) to the ventral longitudinal muscles of
the body. This dorsal cut-off part might be called the myotome, in
the same sense as the corresponding part of the procoelom in the
vertebrate is called the myotome. In both cases the muscles derived
from it form only a part of the voluntary musculature of the animal,
and in both cases the muscles in question are the dorsal longitudinal
muscles of the body, to which must be added the dorso-ventral body-
muscles. Now, the whole of my theory of the origin of vertebrates
arose from the investigation of the structure of the cranial nerves,
which led to the conception that their grouping is not, like the
spinal, a dual grouping of motor and sensory elements, but a dual
grouping to supply two sets of segments, characterized especially by
the different embryological origin of their musculature. The one set
I called the somatic segmentation, because the muscles belonging to
it were the great longitudinal body-muscles ; the other I called the
splanchnic segmentation, because its muscles were those connected
with the branchial and visceral arches. According to my theory,
this latter segmentation was due to the segmentation of the appen-
dages in the invertebrate ancestor ; and in previous chapters, dealing
as they do with the cranial region, attention was especially directed
to the way in which the position of the striated splanchnic muscula-
ture could be explained by a transformation of the prosomatic and
mesosomatic appendages. Now, I am dealing with the metasomatic
region, in which it is true the appendages take a very subordinate
place, but still something corresponding to the splanchnic segments
of the cranial region might fairly be expected to exist, and I therefore

412 THE ORIGIN OF VERTEBRATES

desire to emphasize what appears to me to be the fact, that the
musculature, which in the region of the trunk would correspond to
that derived from the ventral segmentation of the mesoblast in the
region of the head, may have arisen not only from the musculature of
the appendages, but also from the ventral longitudinal musculature
of the body of the invertebrate ancestor, for it seems probable that
this latter musculature had nothing to do with the origin of the great
longitudinal muscles of the vertebrate body, either dorsal or ventral.
The way in which I imagine the obliteration of the atrial cavity
to have taken place is indicated in Fig. 160, B, which is a modifica-
tion of a section across a trilobite-like animal as represented in
Fig. 160, A. As is seen, the pleural folds on each side have nearly
met the bulged-out ventral body-surface. A continuation of the
same process would give Fig. 160, C, which is, to all intents and
purposes, the same as Fig. 159, C, taken from van Wijke, and shows
how the segmental duct is left in the remains of the atrial cavity.
The lining walls of the atrial cavity are represented very black, in
order to indicate the presence of pigment, as indeed is seen in the
corresponding position in Ammoccetes. In these diagrams I have
represented the median ventral surface as a large bulged-out bag,
without indicating any structures in it except the ventral extension
of the proccelom to form the metaccelom. At present I will leave
the space between the central uervous system and the ventral mesen-
tery blank, as in the diagrams ; in my next chapter I will discuss
the possible method of formation within this blank space of the
notochord and midgut. Boveri considers that the obliteration of the
atrial cavity in the higher vertebrates is not complete, but that its
presence is still visible in the shape of the pronephric duct. The
evidence of Maas and others that the duct is formed by the fusion
of the pronephric tubules is, it seems to me, conclusive against
Boveri's view ; but yet, as may be seen from my diagrammatic figures,
the very place where one would expect to find the last remnant
of the atrial cavity is exactly where the pronephric duct is situated.
For my own part I should expect to find evidence of a former
existence of an atrial cavity rather in the pigment round the prone-
phros and its duct than in the duct itself.

The conception that Amphioxus shows us how to account for the
great envelope of somatic muscles which wraps round the vertebrate
body, in that the ancestor of the vertebrate possessed on each side of

THE REGION OF THE SPIXAL CORD

413

-Mes

-My
-Sd

-Met

VMes

Fig. 160. — A, Diagram of Section through a Trilobite-like Animal ; B,
Diagram to illustrate Suggested Obliteration of Appendages and the
Formation of an Atrial Cavity by the Ventral Extension of the
Pleural Folds ; C, Diagram to illustrate the Completion of the Verte-
brate Type by the Meeting of the Pleural Folds in the Mid-ventral
Line and the Obliteration of the Atrial Cavity.

.1/., alimentary canal; N., nervous system; My., myotome; PL, pleuron ; App.,
appendage; Nepli., nephrocoele ; Met., metaccele ; S.d., segmental duct; At.,
atrial chamber; V.Mes., ventral mesentery; Mes., mesonephros. The dotted
line represents the splanchnopleuric mesoblast in all figures.

414 THE ORIGIN OF VERTEBRATES

the body a segmented pleuron, is exactly in accordance with the
theory of the origin of vertebrates deduced from the study of
Ammoccetes, as already set forth in previous chapters. For we see
that one of the striking characteristics of such forms as Bunodes,
Hemiaspis, etc , is the presence of segmented pleural flaps on each
side of the main part of the body ; and if we pass further back to the
great group of trilobites, we find in the most manifold form, and
in various degrees of extent, the most markedly segmented pleural
folds. In fact, the hypothetical figure (Fig. 160, A) which I have
deduced from the embryological evidence, might very well represent
a cross-section of a trilobite, provided only that each appendage of
the trilobite possessed an excretory coxal gland.

The earliest fishes, then, ought to have possessed segmented
pleural folds, which were moved by somatic muscles, and enveloped
the body after the fashion of Ammocoetes and Amphioxus, and I
cannot help thinking that Cephalaspis shows, in this respect also, its
relation to Ammoccetes. It is well known that some of the fossil
representatives of the Cephalaspids show exceedingly clearly that
these animals possessed a very well-segmented body, and it is equally
recognized that this skeleton is a calcareous, not a bony skeleton,
and does not represent vertebrse, etc. It is generally called an
aponeurotic skeleton, meaning thereby that what is preserved repre-
sents not dermal plates alone, or a vertebrate skeleton, but the calcified
septa or aponeuroses between a number of muscle-segments or
myomeres, precisely of the same kind as the septa between the
myomeres in Ammoccetes. The termination of such septa on the
surface would give rise to the appearance of dermal plates or scutes,
or the septa may even have been attached to something of the nature
of dermal plates. The same kind of picture would be represented if
these connective tissue dissepiments of Ammocoetes were calcified,
and the animal then fossilized. In agreement with this interpre-
tation of the spinal skeleton of Cephalaspis, it may be noted that
again and again, in parts of these dissepiments, I have found in old
specimens of AmmocGetes nodules of cartilage formed, and at trans-
formation it is in this very tissue that the spinal cartilages are
formed.

Now, the specimens of Cephalaspis all show, as seen in Fig. 161,
that the skeletal septa cover the body regularly, and then along one
line are bent away from the body to form, as it were, a fringe, or

THE REGION OE THE SPINAL CORD

415

rather a free pleuron, which has been easily pushed at an angle to
the body-skeleton in the process of fossilization. Patten thinks that
this fringed appearance is evidence of a number of segmental appen-
dages which were jointed to the corresponding body-segments, and in
the best specimen at the South Kensington Natural History Museum
he thinks such joints are clearly visible. He concludes, therefore,

Fig. 161.— A, Facsimile of Woodward's Drawing of a Specimen of Cephalaspis
Murchisoni, as seen from the side. The Cephalic Shield is on the
Right and Caudal to it the Pleural Fringes are well shown ; B,
Another Specimen of Cephalaspis Murchisoni taken from the same block
of Stone, showing the Dermoseptal Skeleton and in one place the
Pleural Fringes, be.

that the cephalaspids were arthropods, and not vertebrates. I have
also carefully examined this specimen, and do not consider that what
is seen resembles the joint of an arthropod appendage ; the appear-
ance is rather such as would be produced if the line of attachment of
Patten's appendages to the body were the place where the pleural
body folds became free from the body, and so with any pressure a

4i6

THE ORIGIN OF VERTEBRATES

bending or fracture of the calcified plates would take place along
this line. There is, undoubtedly, an appearance of finish at the
termination of these skeletal friuges, as though they terminated in
a definitely shaped spear-like point, just as is seen in the trilobite
pleura?. This, again, to my mind, is rather evidence of pleural fringes
than of true appendages.

As already argued, I look upon Ammoccetes as the only living
fish at all resembling the cephalaspids ; it is therefore instructive to
compare the arrangement of this spinal dermo-septal skeleton of
Cephalaspis with that of the septa between the myomeres in the

Fig. 162. — A, Arrangement of Septa in Ammoccetes (NC, position of notochord) ;
B, Arrangement op Septa in Amphioxus.

trunk-region of Ammoccetes and Amphioxus. Such a skeleton in
Ammoccetes would be represented by a series of plates overlapping
each other, arranged as in Fig. 162, A, and in Amphioxus as in
Fig. 162, B. I have lettered the corresponding parts of the two
structures by similar letters, a, b, c. Ammoco^tes differs in configu-
ration from Amphioxus in that it possesses an extra dorsal (a, d) and
an extra ventral bend. Ammoccetes is a much rounder animal than
Amphioxus, and both the dorsal and ventral bends are on the extreme
ventral and dorsal surfaces — surfaces which can hardly be said to
exist in Amphioxus. The part, then, of such an aponeurotic skeleton

THE REGION OF THE SPINAL CORD 417

in Ammoccetes which I imagine corresponds to b, c in Amphioxus,

and therefore would represent the pleural fold, is the part ventral to
the bend at b. In both the animals this bend corresponds to the
position of the notochord NC.

The skeleton of Cephalaspis compares more directly with that of
Ammocoetes than that of Amphioxus, for there is the same extra
dorsal bend (Fig. 161, a, d) as in Ammoccetes; the lateral part of the
skeleton again gives an angle a, b, c ; the part from b to c would
therefore represent the pleural fold. I picture to myself the sequence
of events somewhat as follows : —

First, a protostracan ancestor, which, like Peripatus, possessed
appendages on every segment into which ccelomic diverticula passed,
forming a system of coxal glands ; such glands, being derived from
the segmental organs of the Chsetopoda, discharged originally to the
exterior by separate openings on each segment. It is, however,
possible, and I think probable, that a fusion of these separate ducts
had already taken place in the protostracan stage, so that there was
only one external opening for the whole of these metasomatic coxal
glands, just as there is only one external opening for the correspond-
ing prosomatic coxal glands of Limulus. Then, by the ventral growth
of pleural body-folds, such appendages became enclosed and useless,
and the coxal glands of the post-branchial segments, with their
segmental or pronephric duct, were all that remained as evidence of
such appendages. This dwindling of the metasomatic appendages
was accompanied by the getting-rid of free appendages generally, in
the manner already set forth, with the result that a smooth fish-like
body-surface was formed ; then the necessity of increasing mobility
brought about elongation by the addition of segments between those
last formed and the cloacal region. Each of such new-formed
segments was appendageless, so that its segmental organ was not a
coxal gland, but entirely somatic in position, and formed, therefore, a
mesonephric tubule, not a pronephric one. Such glands could no
longer excrete to the exterior, owing to the enclosing shell of the
pleural folds ; but the pronephric duct was there, already formed,
and so these nephric tubules opened into that, instead of, as in the
case of the branchial slits, forcing their way through the pleural
walls when the atrium became closed.

2 E

41 8 THE ORIGIN OF VERTEBRATES

The Meaning ok the Ductless Glands.

If it is a right conception that the excretory organs of the proto-
stracan group, which gave origin to the vertebrates as well as to the
crustaceans and arachnids, were of the nature of coxal glands, then it
follows that such coxal glands must have existed originally on every
segment, because they themselves were derived from the segmental
organs of the annelids ; it is therefore worth while making an attempt
to trace the fate of such segmental organs in the vertebrate as well
as in the crustacean and arachnid.

Such an attempt is possible, it seems to me, because there exists
throughout the animal kingdom striking evidence that excretory
organs which no longer excrete to the exterior do not disappear, but
still perform excretory functions of a different character. Their cells
still take up effete or injurious substances, and instead of excreting
to the exterior, excrete into the blood, forming either ductless
glands of special character, or glands of the nature of lymphatic
glands.

The problem presented to us is as follows : —

The excretory organs of both arthropods and vertebrates arose
from those of annelids, and were therefore originally present in every
segment of the body. In most arthropods and vertebrates they are
present only in certain regions ; in the former case, as the coxal glands
of the prosomatic or head-region ; in the latter, as the nephric glands
of the metasomatic or trunk-region, and, in the case of Amphioxus, of
the mesosomatic or branchial region.

In the original arthropod, judging from Peripatus, they were
present, as in the annelid, in all the segments of the body, and
formed coxal glands. Therefore, in the ancestors of the living
Crustacea and Arachnida, coxal glands must have existed in all the
segments of the body, and we ought to be able to find the vestiges
of them in the mesosomatic or branchial and metasomatic or
abdominal regions of the body.

Similarly, in the vertebrates, derived, as has been shown, not
from the annelids, but from an arthropod stock, evidence of the
previous existence of coxal glands ought to be manifested in the
prosomatic or trigeminal region, in the mesosomatic or branchial
region, as well as in the metasomatic or post-branchial region.

How does an excretory organ change its character when it ceases

THE REGION OF THE SPINAL CORD 419

to excrete to the exterior ? What should we look for in our search
after the lost coxal glands ?

The answer to these questions is most plainly given in the case
of the pronephros, especially in Myxine, where Maas has been able
to follow out the whole process of the conversion of nephric tubules
into a tissue resembling that of a lymph-gland.

He states, in the first place, that the pronephros possesses a
capillary network, which extends over the pronephric duct, while
the tubules of the mesonephros possess not only this capillary net-
work, equivalent to the capillaries over the convoluted tubules in the
higher vertebrates, but also a true glomerulus, in that the nephric
segmental arteriole forms a coil (Knauel), and pushes in the wall of
the mesonephric tubule. He describes the pronephros of large adult
individuals as consisting of —

1. Tubules with funnels which open into the pericardial ccelom.

2. A large capillary network (the glomus) at the distal end.

3. A peculiar tissue (the ' strittige Gewebe ' of the Semon-Spengel
controversy), which Spengel considers to be composed of the altered
epithelium of pronephric tubules, while Semon looks on it as an
amalgamation of glomeruli.

Maas is entirely on the side of Spengel, and shows that this
peculiar tissue is actually formed by modified pronephric tubules,
which become more and more lymphatic in character.

He says : " The pronephros consists of a number of nephric
tubules, placed separately one behind the other, which were origi-
nally segmental in character, each one of which is supplied by a
capillary network from a segmental branch of the aorta. The
tubules begin with many mouths (dorso-lateral and medial- ventral)
in the pericardial cavity ; on their other blind end they have lost
their original external opening, and there, in the cranial portion of
the head-kidney, before they have joined together to form a collecting
duct, they, together with the vascular network, are transformed into
a peculiar adrenal-like tissue. The most posterior of the segmental
capillary nets retain their original character, and are concentrated
into the separate capillary mass known as the glomus."

Later on he says : " Further, the separate head-kidney is more and
more removed in structure from an excretory organ in the ordinary
sense. One cannot, however, speak of it as an organ becoming rudi-
mentary ; this is proved not only by the progressive transformation

420 THE ORIGIN OF VERTEBRATES

of its internal tissue into a tissue of a very definite character,
but also by the cilia in its canals, and the steady increase in the
number of its funnels. It appears, therefore, to be the conversion of
an excretory organ into an organ for the transference of fluid out of
the coelom into a special tissue, i.e. into its blood-sinus ; in other
words, into an organ which must be classed as belonging to the
lymph-system."

In exact correspondence with this transformation of a nephric
tubule into a ductless gland of the nature of a lymphatic gland, is the
formation of the head-kidney in the Teleostea. Thus, Weldon points
out that, though the observations of Balfour left it highly probable
that the " lymphatic " tissue described by him was really a result of
the transformation of part of the embryonic kidney, he did not inves-
tigate the details of its development. This was afterwards done by
Emery, with the following results : " In those Teleostea which he
has studied, Professor Emery finds that at an early stage the kidney
consists entirely of a single pronephric funnel, opening into the
pericardium, and connected with the segmental duct, which already
opens to the exterior. Behind this funnel, the segmental duct is
surrounded by a blastema, derived from the intermediate cell-mass,
which afterwards arranges itself more or less completely into a series
of solid cords, attaching themselves to the duct. These develop a
lumen, and become normal segmental tubules, but it is, if I may be
allowed the expression, a matter of chance how much of the blastema
becomes so transformed into kidney tubules, and how much is left
as the ' lymphatic ' tissue of Balfour, this ' lymphatic ' tissue remain-
ing either in the pronephros only, or in both pro- and neso-nephros."

If we turn now to the invertebrates, we see also how close a con-
nection exists between lymphatic and phagocytic organs and excretory
organs. The chief merit for this discovery is due to Kowalewsky,
who, taking a hint from Heidenhain's work on the kidney, in which
he showed how easy it was to find out the nature of different parts of
the mammalian excretory organ by the injection of different sub-
stances, such as a solution of ammoniated carmine, or of indigo-
carmine, has injected into a large number of different invertebrates
various colouring matters, or litmus, or bacilli, and thus shown the
existence, not only of known excretory organs, but also of others,
lymphatic or lymphoid in nature, not hitherto suspected.

In all cases he finds that a phagocytic action with respect to solid

THE REGION OF THE SPINAL CORD \2\

bodies is a property of the leucocytes, and that these leucocytes which
are found in the coelomic spaces of the Annelida, etc., are apparently
derived from the epithelium of such spaces. Also by the prolifera-
tion of such epithelium in places, e.g. the septal glands of the terres-
trial Oligochreta, segmental glandular masses of such tissue are
formed which take up the colouring matter, or the bacilli. In the
limicolous Oligochseta such septal glands are not found, but at the com-
mencement of the nephridial organ, immediately following upon the
funnel, a remarkable modification of the nephridial wall takes place to
form a large cellular cavernous mass, the so-called filter, which in
Euaxes is full of leucocytes ; the cells are only definable by their nuclei,
and look like and act in the same way as the free leucocytes outside
this nephridial appendage. As G. Schneider points out, the whole
arrangement is very like that described by Kowalewsky in the
leeches Clepsine and Nephelis, where, also immediately succeeding
the funnel of the nephridial organ, a large accessory organ is found,
which is part of the nephridium, and is called the nephridial capsule.
This is the organ par excellence which takes up the solid carmine-
grains and bacilli, and apparently, from Kowalewsky's description,
contains leucocytes in large numbers. We see, then, that in such
invertebrates, just as in the vertebrate, modifications of the true excre-
tory organ may give rise to phagocytic glands of the nature of lym-
phatic glands. Further, these researches of Kowalewsky suggest in
the very strongest manner that whenever by such means new, hitherto
unsuspected glands are discovered, such glands must belong to the
excretory system, i.e. must be derived from ccelomic epithelium,
even when all evidence of any cceloin has disappeared. Kowalewsky
himself was evidently so impressed with the same feeling that he
heads one of his papers " The Excretory Organs of the Pantopoda,"
although the organs in question had been discovered by him by this
method, and appeared as ductless glands with no external opening.

To my mind these observations of Kowalewsky are of exceeding-
interest, for it is immediately clear that if the segmental organs of
the annelids, which must have existed on all the segments of the
forefathers of the Crustacea and Arachnida (the Protostraca), have left
any sign of their existence in living crustaceans and arachnids, then
such indication would most likely take the form of lymphatic glands
in the places where the excretory organs ought to have been.

Now, as already pointed out in Peripatus, such segmental organs

42 2 THE ORIGIN OF VERTEBRATES

were formed by the ventral part of the coelom, and dipped originally
into each appendage. We know also that each segment of an arachnid
embryo possesses a crelomic cavity in its ventral part which extends
into the appendage on each side ; this cavity afterwards disappears,
and is said to leave no trace in the adult of any excretory coxal gland
derived from its walls. If, however, it is found that in the very
position where such organ ought to have been formed a segmentally
arranged ductless gland is situated, the existence of which is shown
by its taking up carmine, etc., then it seems to me that in all
probability such gland is the modification of the original coxal gland.

This is what Kowalewsky has done. Thus he states that
Metschnikoff had fed My sis with carmine-grains, and found tubules
at the base of the thoracic feet coloured red with carmine. He him-
self used an allied species, Parapodopsis cornutum, and found here
also that the carmine was taken up by tubules situated in the basal
segments of the feet. In Nebalia, feeding experiments with alizarin
blue and carmine stained the antennal glands, and showed the
existence of glands at the base of the eight thoracic feet. These
glands resemble the foot-glands of Mysis, Parapodopsis, and Pake-
mon, and lie in the space through which the blood passes from the
thoracic feet, i.e. from the gills, to the heart. In Squilla also, in
addition to the shell-glands, special glands were discovered on the
branchial feet on the path of the blood to the heart. These glands
form continuous masses of cells which constitute large compact glands
at the base of the branchial feet. Single cells of the same sort are
found along the whole course of the branchial venous canal, right
up to the pericardium.

These observations show that the Crustacea possess not only true
excretory organs in the shape of coxal glands, i.e. antennary glands,
shell-glands, etc., in the cephalic region, but also a series of segmental
glands situated at the base of the appendages, especially of the respi-
ratory appendages : a system, that is to say, of coxal glands which
have lost their excretory function, through having lost their external
opening, but have not in consequence disappeared, but still remain
in situ, and still retain an important excretory function, having
become lymphatic glands containing leucocytes. Such glands are
especially found in the branchial appendages, and are called branchial
glands by Cuenot, who describes them for all Decapoda.

Further, it is significant that the same method reveals the

THE REGION OF THE SPINAL CORD 423

existence in Pantopoda of a double set of glands of similar character,
one set in the basal segments of the appendage, and the other in
the adjacent part of the body.

In scorpions also, Kowalewsky has shown that the remarkable
lymphatic organ situated along the whole length of the nerve-cord in
the abdominal region takes up carmine grains and bacilli ; an organ
which in Androctonus does not form one continuous gland, but a
number of separate, apparently irregularly grouped, glandular bodies.

In addition to this median lymphatic gland, Kowalewsky has
discovered in the scorpion a pair of lateral glands, to which he gives
the name of lymphoid glands, which communicate with the thoracic
body-cavity {i.e. the pseudocode), are phagocytic, and, according to
him, give origin to leucocytes by the proliferation of their lining
cells, thus, as he remarks, reminding us of the nephridial capsules
of Clepsine. These glands are so closely related in position to the
coxal glands on each side that he has often thought that the lumen
of the gland communicated with that of the coxal gland ; he, how-
ever, has persuaded himself that there is no true communication
between the two glands. Neither of these organs appears to be
segmental, and until we know how they are developed it is not
possible to say whether they represent fused segmental organs or not.

The evidence, then, is very strong that in the Crustacea and
Arachnida the original segmental excretory organs do not disappear,
but remain as ductless glands, of the nature of lymphatic glands,
which supply leucocytes to the system.

Further, the evidence shows that the nephric organs, or parts of
the ccelom in close connection with these organs, maybe transformed
into ductless glands, which do not necessarily contain free leucocytes
as do lymph-glands, but yet are of such great importance as excretory
organs that their removal profoundly modifies the condition of the
animal. Such a gland is the so-called adrenal or suprarenal body,
disease of which is a feature of Addison's disease ; a gland which
forms and presumably passes into the blood a substance of remark-
able power in causing contraction of blood-vessels, a substance which
has lately been prepared in crystalline form by Jokichi Takamine,
and called by him " adrenalin " ; a gland, therefore, of very distinctly
peculiar properties, which cannot be regarded as rudimentary, but is
of vital importance for the due maintenance of the healthy state.

In the Elasmobranchs two separate glandular organs have been

424 THE ORIGIN OF VERTEBRATES

called suprarenal ; a segmental series of paired organs, each of which
possesses a branch from the aorta and a sympathetic ganglion, and an
unpaired series in close connection with the kidneys, to which Balfour
gave the name of interrenal glands. Of these two sets of glands,
Swale Vincent has shown that the extract of the interrenals has no
marked physiological effect, in this respect resembling the extract of
the cortical part of the mammalian gland, while the extract of the
paired segmental organs of the Elasmobranch produces the same
remarkable rise of blood- pressure as the extract of the medullary
portion of the mammalian gland.

The development also of these two sets of glands is asserted to be
different. Balfour considered that the suprarenals were derived from
sympathetic ganglion-cells, but left the origin of the interrenals
doubtful. Weldon showed that the cortical part of the suprarenals
in the lizard was derived from the wall of the glomerulus of a
number of mesonephric tubules. In Pristiurus, he stated that the
mesoblastic rudiment described by Balfour as giving origin to the
interrenals is derived from a diverticulum of each segmental tubule,
close to the narrowing of its funnel-shaped opening into the body-
cavity. With respect to the paired suprarenals he was unable to
speak positively, but doubted whether they were derived entirely
from sympathetic ganglia.

Weldon sums up the results of his observations by saying :
" That all vertebrates except Amphioxus have a portion of the
kidney modified for some unknown purpose not connected with
excretion ; that in Cyclostomes the pronephros alone is so modified,
in Teleostei the pro- and part of the meso-nephros ; while in the
Elasmobranchs and the higher vertebrates the mesonephros alone
gives rise to this organ, which has also in these forms acquired a
secondary connection with certain of the sympathetic ganglia."

Since Weldon's paper, a large amount of literature on the origin
of the adrenals has appeared, a summary of which, up to 1891, is
given by Hans Eabl in his paper, and a further summary by Aichel
in his paper published in 1900. The result of the investigations up
to this latter paper may be summed up by saying that the adrenals,
using this term to include all these organs of whatever kind, are in
all cases, partly at all events, derived from some part of the walls of
either the mesonephric or pronephric excretory organs, but that in
addition a separate origin from the sympathetic nervous system must

THE REGION OF THE SPINAL CORD 425

be ascribed to the medullary part of the organ and to the separate
paired organs in the Elasmobranchs, which are equivalent to the
medullary part in other cases.

The evidence, then, of the transformation of the known vertebrate
excretory organs — the pronephros and the mesonephros — leads to the
conclusion that in our search for the missing coxal glands of the
meso- and pro-somatic regions, we must look for either lymphatic
glands, or ductless glands of distinct importance to the body. I have
already considered the question in the prosomatic region, and have
given my reasons why the pituitary gland must be looked upon as
the descendant of the arthropod coxal gland. In this case also the
resulting ductless gland is still of functional importance, for disease of
it is associated with acromegaly. If, as is possible, it is homologous
with the Ascidian hypophysial gland, then it is confirmatory evidence
that this latter is said by Julin to be an altered nephridial organ.

Finally, I come to the mesosomatic or branchial region ; and here,
strikingly enough, we find a perfectly segmental glandular organ of
mysterious origin — the thymus gland — segmental with the branchice,
not necessarily with the myotomes, belonging, therefore, to the appen-
dicular system ; and since the branchiae represent, according to my
theory, the basal part of the appendage, such segmental glands would
be in the position of coxal glands. Here, then, in the thymus may
be the missing mesosomatic coxal glands.

What, then, is the thymus ?

The answer to this question has been given recently by Beard,
who strongly confirms Kolliker's original view that the thymus is a
gland for the manufacture of leucocytes, and that such leucocytes are
directly derived from the epithelial cells of the thymus. Kolliker
also further pointed out that the blood of the embryo is for a certain
period destitute of leucocytes. Beard confirms this last statement,
and says that up to a certain stage (varying from 10 to 16 mm. in
length of the embryo) the embryos of Raja batis have no leucocytes
in the blood or elsewhere. Up to this period the thymus-placode is
well formed, and the first leucocytes can be seen to be formed in it
from its epithelial cells ; then such formation takes place with great
rapidity, and soon an enormous discharge of leucocytes occurs from
the thymus into the tissue-spaces and blood. He therefore concludes
that all lymphoid tissues in the body arise originally from the thymus
gland, i.e. from leucocytes discharged from the thymus.

426 THE ORIGIN OF VERTEBRATES

The segmental branchial glands, known by the name of thymus,
are, according to this view, the original lymphatic glands of the
vertebrate ; and it is to be noted that, in fishes and in Amphibia,
lymphatic glands, such as we know them in the higher mammals, do
not exist ; they are characteristic of the higher stages of vertebrate
evolution. In the lower vertebrates, the only glandular masses
apart from the cell-lining of the body-cavity itself, which give rise
to leucocyte-forming tissue, are these segmental branchial glands, or
possibly also the modified post-branchial segmental glands, known as
the head-kidney in Teleostea, etc.

The importance ascribed by Beard to the thymus in the forma-
tion of leucocytes in the lowest vertebrates would be considerably
reduced in value if the branchial region of Ammoccetes possessed
neither thymus glands nor anything equivalent to them. Such,
however, is not the case. Schaffer has shown that in the young
Ammoccetes masses of lymphatic glandular tissue are found segmen-
tary arranged in the neighbourhood of each gill-slit — tissue which
soon becomes converted into a swarming mass of leucocytes, and
shows by its staining, etc., how different it is from a blood-space.
The presence of this thymus leucocyte-forming tissue, as described
by Schaffer, is confirmed by Beard, and I myself have seen the same
thing in my youngest specimen of Ammoccetes.

Further, the very methods by which Kowalewsky has brought
to light the segmental lymph-glands of the branchial region of the
Crustacea, etc., are the same as those by which Weiss discovered the
branchial nephric glands in Amphioxus — excretory organs which
Boveri considers to represent the pronephros of the Craniota. In
this supposition Boveri is right, in so far that both pronephros and
the tubules in Amphioxus belong to the same system of excretory
organs; but I entirely agree with van Wijhe that the region in
Amphioxus is wrong. The tubules in Amphioxus ought to be repre-
sented in the branchial region of the Craniota, not in the post-
branchial region ; van Wijhe therefore suggests that further researches
may homologize them with the thymus gland in the Craniota, not
with the pronephros. This suggestion of van Wijhe appears to me
a remarkably good one, especially in view of the position of the
thymus glands in Ammoccetes and the nephric branchial glands
in Amphioxus. If, as I have pointed out, the atrial cavity of
Amphioxus has been closed in Ammoccetes by the apposition of

THE REGION OF THE SPINAL CORD 427

the pleural fold with the branchial body-surface, then the remains of
the position of the atrial chamber must exist in Ammoccetes as that
extraordinary space between the somatic muscles and the branchial
basket-work filled with blood-spaces and modified muco-cartilage. It
is in this very space, close against the gill -slits, that the thymus
glands of Ammoccetes are found, in the very place where the nephric
tubules of Amphioxus would be found if its atrial cavity were closed
completely. Instead, therefore, of considering with Boveri that the
branchial nephric tubules of Amphioxus still exist in the Craniota
as the pronephros, and that the atrial chamber has narrowed down to
the pronephric duct, I would agree with van Wijhe that the pro-
nephros is post-branchial, and suggest that by the complete closure
of the atrial space in the branchial region the branchial nephric
tubules have lost all external opening, and consecpuently, as in all
other cases, have changed into lymphatic tissue and become the
segmental thymus glands.

As van Wijhe himself remarks, the time is hardly ripe for making
any positive statement about the relationship between the thymus
gland and branchial excretory organs. There is at present not suffi-
cient consensus of opinion to enable us to speak with any certainty
on the subject, yet there is so much suggestiveness in the various
statements of different authors as to make it worth while to consider
the question briefly.

On the one hand, thymus, tonsils, parathyroids, epithelial cell-
nests, and parathymus, are all stated to be derivatives of the epithelium
lining the gill-slits, and Maurer would draw a distinction between
the organs derived from the dorsal side of the gill-cleft and those
derived from the ventral side — the former being thymus, the latter
forming the epithelial cell-nests, i.e. parathyroids. The thymus in
Ammocujtes, according to Schaffer, lies both ventral and dorsal to the
gill-cleft ; Maurer thinks that only the dorsal part corresponds to
the thymus, the ventral part corresponding to the parathyroids, etc.
Structurally, the thymus, parathyroids, and the epithelial cell-nests
are remarkably similar, so that the evidence appears to point to the
conclusion that, in the neighbourhood of the gill-slits, segmentally
arranged organs of a lymphatic character are situated, which give
origin to the thymus, parathyroids, tonsils, etc. Now, among these
organs, i.e. among those ventrally situated, Maurer places the
carotid gland, so that, if he is right, the origin of the carotid gland

428 THE ORIGIN OF VERTEBRATES

might be expected to help in the elucidation of the origin of the
thymus.

The origin of the carotid gland has been investigated recently by
Kohn, who finds that it is associated with the sympathetic nervous
system in the same way as the suprarenals. He desires, in fact, to
make a separate category for such nerve-glands, or paraganglia, as he
calls them, and considers them all to be derivatives of the sympathetic
nervous system, and to have nothing to do with excretory organs. The
carotid gland is, according to him, the foremost of the suprarenal
masses in the Elasmobranchs, viz. the so-called axillary heart.

In my opinion, nests of sympathetic ganglion-cells necessarily
mean the supply of efferent fibres to some organ, for all such ganglia
are efferent, and also, if they are found in the organ, would have been
brought into it by way of the blood-vessels supplying the organ, so
that Aichel's statement of the origin of the suprarenals in the
Elasmobranchs seems to me much more probable than a derivation
from nerve-cells. If, then, it prove that Aichel is right as to the
origin of the suprarenals, and Kohn is right in classifying the carotid
gland with the suprarenals, then Maurer's statements would bring
the parathyroids, thymus, etc., into line with the adrenals, and sug-
gest that they represent the segmented glandular excretory organs of
the branchial region, into which, just as in the interrenals of Elasmo-
branchs, or the cortical part of the adrenals of the higher vertebrates,
there lias been no invasion of sympathetic ganglion-cells.

Wheeler makes a most suggestive remark in his paper on Petro-
myzon : he thinks he has obtained evidence of serial homologues of
the pronephric tubules in the branchial region of Ammoccetes, but
has not been able up to the present to follow them out. If what
he thinks to be serial homologues of the pronephric tubules in
the branchial region should prove to be the origin of the thymus
glands of Schaffer, then van Wijhe's suggestion that the thymus
represents the excretory organs of the branchial region would
gain enormously in probability. Until some such further investiga-
tion has been undertaken, I can only say that it seems to me most
likely that the thymus, etc., represent the lymphatic branchial glands
of the Crustacea, and therefore represent the missing coxal glands of
the branchial region.

This, however, is not all, for the appendages of the mesosomatic
region, as I have shown, do not all bear branchire ; the foremost or

THE REGION OF THE SPINAL CORD 429

opercular appendage carries the thyroid gland. Again, the basal part
of the appendage is all that is left ; the thyroid gland is in position a
coxal gland. It ought, therefore, to represent the coxal gland of this
appendage, just as the thymus, tonsils, etc., represent the coxal glands
of the rest of the mesosoniatic appendages. In the thyroid gland we
again see a ductless gland of immense importance to the economy,
not a useless organ, but one, like the other modified coxal glands,
whose removal involves far-reaching vital consequences. Such a
gland, on my theory, was in the arthropod a part of the external genital
ducts which opened on the basal joint of the operculum. What, then,
is the opinion of morphologists as to the meaning of these external
genital ducts ?

In a note to Gulland's paper on the coxal glands of Limulus,
Lankester states that the conversion of an externally-opening tubular
gland (coxal gland) into a ductless gland is the same kind of thing
as the history of the development of the suprarenal from a modified
portion of mesonephros, as given by Weldon. Further, that in other
arthropods with glands of a tubular character opening to the exterior
at the base of the appendages, we also have coxal nephridia, such
as the shell-glands of the Entomostraca, green glands of Crustacea
(antennary coxal gland) ; and further on he writes : " When once the
notion is admitted that ducts opening at the base of limbs in the
Arthropoda are possibly and even probably modified nephridia, we
immediately conceive the hypothesis that the genital ducts of the
Arthropoda are modified nephridia."

So, also, Korschelt and Heider, in their general summing up on
the Arthropoda, say : " In Peripatus, where the nephridia appear, as
in the Annelida, in all the trunk-segments, a considerable portion of
the primitive segments is directly utilized for the formation of the
nephridia. In the other groups, the whole question of the rise of
the organs known as nephridia is still undecided, but it may be
mentioned as very probable that the salivary and anal glands of
I'eripatus, the antennal and shell-glands of the Crustacea, the coxal
glands of Limulus and the Arachnida, as well as the efferent genital
ducts, are derived from nephridia, and in any case are mesodermal
in origin."

The necessary corollary to this exceedingly probable argument is
that glandular structures such as the uterine glands of the scorpion
already described, which are found in connection with these terminal

43° THE ORIGIN OF VERTEBRATES

genital ducts, may be classed as modified nephridial glands, and that
therefore the thyroid gland of Ammocoetes, which, on the theory of
this book, arose in connection with the opercular genital ducts of the
paheostracan ancestor, represents the coxal glands of this fused pair
of appendages. Such a gland, although its function in connection
with the genital organs had long disappeared, still, in virtue of its
original excretory function, persisted, and even in the higher verte-
brates, after it had lost all semblance of its former structure and
become a ductless gland of an apparently rudimentary nature, still,
by its excretory function, demonstrates its vital importance even to
the highest vertebrate.

By this simple explanation we see how these hitherto mysterious
ductless glands, pituitary, thymus, tonsils, thyroid, are all accounted
for, are all members of a common stock — coxal glands— which origi-
nally, as in Peripatus, excreted at the base of the prosomatic and
mesosomatic appendages, and are still retained because of the impor-
tance of their excretory function, although ductless owing to the
modification of their original appendages.

Finally, there is yet another organ in the vertebrate which follows
the same law of the conversion of an excretory organ into a lymphatic
organ when its connection with the exterior is obliterated, and that is
the vertebrate body-cavity itself. According to the scheme here put
forth, the body-cavity of the vertebrate arose by the fusion of a
ventral prolongation of the original nephrocele on each side ; pro-
longations which accompanied the formation of the new ventral mid-
gut, and by their fusion formed originally a pair of cavities along the
whole length of the abdomen, being separated from each other by the
ventral mesentery of the gut. Subsequently, by the ventral fusion of
these two cavities, the body-cavity of the adult vertebrate was formed.

This is simply a statement of the known method of formation of
the body-cavity in the embryo, and its phylogenetic explanation is
that the body-cavity of the vertebrate must be looked upon as a
ventral prolongation of the original ancestral body-cavity. Embryo-
logy clearly teaches that the original body-cavity or somite was
confined to the region of the notochord and central nervous system,
and there, just as in Peripatus, was divisible into a dorsal part, giving
origin to the myoccele, and a ventral part, forming the nephrocele.
From this original nephroccele are formed the pronephric excretory
organs, the mesonephric excretory organs, and the body-cavity.

THE REGION OF THE SPINAL CORD 43 1

That the vertebrate body-cavity was originally a nephroccele is
generally accepted, and its excretory function is shown by the fact
that it communicates with the exterior in all the lower vertebrates,
either through abdominal pores or by way of nephridial funnels.
Bles has shown how largely these two methods of communicating
with the exterior mutually exclude each other. In the higher verte-
brates both channels become closed, except in the case of the
Fallopian tubes, and thus, so to speak, the body-cavity becomes a
ductless gland, still, however, with an excretory function, but now,
as in all other cases, forming a part of the lymphatic rather than of
the true excretory system.

Summary.

The consideration of the formation of the vertebrate cranial region, as set
forth in previous chapters, indicates that the ancestor of the vertebrates was
not an arachnid purely or a crustacean purely, but possessed partly crustacean
and partly araclinid characters. In order to express this conclusion, I have
used the term Protostraca, invented by Korschelt and Heider, to indicate a
primitive arthropod group, from which both arachnids and crustaceans may be
supposed to have arisen, and have therefore stated that the vertebrate did not
arise directly from the annelids, but from the Protostraca. Such an origin
signifies that the origin of the excretory organs of the vertebrate must not
be looked for in the segmental organs of the annelid, but rather in such
modified annelid org-ans as would naturally exist in a primitive arthropod
group. The nature of such organs may be inferred, owing to the fortunate
circumstance that so primitive an arthropod as Peripatus still exists, and we
may conclude that the protostracan ancestor possessed in every segment a pair
of appendages and a pair of ccelomic cavities, which extended into the base of
these appendages. The ventral portion of each of these ccelomic cavities
separated off from the dorsal and formed a nephrocele, giving- origin to a
segmental excretory organ, which, seeing- that its end-vesicle was in the base
of the appendage, and seeing also the nature of the known arachnid and
crustacean excretory organs, may fitly be termed a coxal gland. This, then,
is the working hypothesis to explain the difficulties connected with the origin
of the pronephros and mesonephros — that the original segmental organs were
coxal glands, and therefore indicated the presence of appendages. This
hypothesis leads to the following conclusions : —

1. The coxal glands belonging- to the post-branchial appendag-es of the
invertebrate ancestor are represented by the pronephric tubules, and existed
over the whole metasomatic region.

1. Such glands discharged into a common duct — the pronephric duct —
which opened into the cloacal region, either in the protostracan stage, when
the metasomatic appendages were still in existence, just as the coxal glands
of the prosomatic region in Limulus discharge into a common duct, or else the
pronephric duct was formed when the appendages were obliterated.

432 THE ORIGIN OF VERTEBRATES

3. The metasomatic appendages disappeared owing- to their enclosure by
pleural folds, which., meeting- in the mid-ventral line, not only caused the
obliteration of the appendages, and gave a smooth fish-like body-surface to the
animal, but also caused the formation of an atrial cavity.

4. Into these pleural folds the dorsal longitudinal muscles of the body
extended, and ultimately reached to the ventral surface, thus forming the
somatic muscles of the vertebrate body.

5. When the pleural folds had met in the mid-ventral line the animal had
became a vertebrate, and was dependent for its locomotion on the movements
of these somatic muscles, and not on the movements of appendages. Conse-
quently, elongation of the trunk-region took place, for the purpose of increasing
mobility, by the formation of new metameres.

6. Each of such metameres possessed its own segmental excretory organ,
formed in the same way as the previous pronephric organs, but, as there were
no appendages in these new-formed segments, the excretory organs took on the
characters of a mesonephros, not a pronephros, and opened into the pronephric
duct, because the direct way to the exterior was blocked by the enveloping
pleural folds.

7. The group of annelids from which the protostracan ancestor of the
vertebrates arose was the highest annelidan group, viz. the Polychada. as
shown by the nature of the excretory organs in Amphioxus.

8. The coxal glands of the protostracan ancestor existed on all the segments,
and were, therefore, divisible into three groups, prosomatic, mesosomatic. and
metasomatic ; these three groups of coxal glands still exist in the vertebrate
as ductless glands.

9. The prosomatic coxal glands form the pituitary body.

10. The mesosomatic coxal glands form the thymus, thyroid, parathyroids,
tonsils, etc.

11. The metasomatic coxal glands form the adrenals.

12. The proccelom of the vertebrate is the proccelom of the protostracan
ancestor, which splits into a dorsal part, the myoccele, and a ventral part, the
nephrocele. This latter part not only forms the pronephros and mesonephros,
but also by a ventral extension gives origin to the walls of the vertebrate body-
cavity or metaccele.

13. This ventral extension of the original nephroccele at first excreted to
the exterior, through abdominal pores, or through peritoneal funnels. When
such paths to the exterior became closed, it also became a ductless gland,
belonging to the lymphatic system.

CHAPTER XIII
THE NOTOCHORD AND ALIMENTARY CANAL

Relationship between notocliord and gut. — Position of unseginented tube of
notochord. — Origin of notocliord from a median groove. — Its function as
an accessory digestive tube. — Formation of notocliordal tissue in inverte-
brates from closed portions of the digestive tube. — Digestive power of the
skin of Ammocoetes. — Fonnation of new gut in Ammocoetes at transforma-
tion.— Innervation of the vertebrate gut. — The three outflows of efferent
nerves belonging to the organic system. — The original close contiguity of
the respiratory chamber to the cloaca. — The elongation of the gut. —
Conclusion.

Int the previous chapters all the important organs of the arthropod
have been found in the vertebrate in their appropriate place, of
similar structure, and innervated from corresponding parts of the
central nervous system. Such comparison is possible only as long-
as the ventral and dorsal surfaces of the vertebrate correspond with
the respective surfaces of the arthropod, and no reversal is assumed.
This method of comparative anatomy is the surest and most
certain guide to the relationship between two animals, and when
the facts obtained by the anatomical method are so strikingly
confirmatory of the paheontological evidence, the combined evidence
becomes so strong as to amount almost to a certainty that vertebrates
did arise from arthropods in the manner mapped out in previous
chapters, and not from a hypothetical group of animals, such as is
postulated in the theory of their origin from forms like Balanoglossus.
The latter theory derives the alimentary canal of the vertebrate
from that of the invertebrate, and finds in the latter the commence-
ment of the notochord. In the comparison which I have made the
alimentary canal of the invertebrate ancestor has become the tube
of the central nervous system of the vertebrate, and there is no sign
of a notochord whatever. All the organs of the arthropod have
already been allocated ; where the notochord is situated in the

2 F

434 THE ORIGIN OF VERTEBRATES

vertebrate there is nothing but a gap in the invertebrate, but the
position of that gap can be settled with great accuracy from
the previous comparison of organs in the two groups. So, also, the
alimentary canal of the vertebrate is from the very nature of the
case a new organ, yet, as has been shown in Chapter V., the com-
parison of the respiratory organs in the two groups gives a strong
suggestion of the manner in which such a canal was formed.

'oov

The Origin of the Notochord.

The time has now come to endeavour to frame a plausible theory
of the method of formation of the notochord and the new alimentary
canal, and thus to complete the diagram on p. 413. The comparative
method is no longer available, for these structures are both unrepre-
sented as such in the arthropod ; any suggested explanation, therefore,
must be more tentative, and cannot give the same feeling of certainty
as is the case with all the organs already considered. Our only chance
of finding out the past history of the notochord lies in the embryo-
logical method, in the hope that, according to the ' law of recapitu-
lation,' the ancestral history may be repeated in the ontogeny with
sufficient clearness to enable some conclusion to be drawn.

At the outset, one point comes out clearly— the close relationship
between the notochord and the vertebrate gut ; they are both derived
from the same layer, both parts of the same structure. On this
point all embryologists are agreed ; it is expressed in such statements
as, "the notochord, as well as the alimentary canal, is formed from
hypoblast " ; " the notochord arises as a thickening in the dorsal wall
of the alimentary canal." The two structures are so closely connected
together that they must be considered together. If we can conjecture
the origin of the one, we may be sure that we have the clue to the
origin of the other. The two together form the one new organ which
distinguishes the vertebrate from the arthropod, the only thing left
which requires explanation for the completion of this strange history.

What, then, is the notochord ? What are its characteristics ? In
the highest vertebrates it is conspicuous only in the embryo ; with
the development of the axial skeleton it is more and more squeezed
out of existence, until in the adult it is no longer visible. By the
' law of recapitulation ' this developmental history implies that, as we
descend the vertebrate phylum, the notochord ought to be more and

THE NOTOCHORD AXD ALIMENTARY CANAL 435

more conspicuous, more and more permanent daring the life of the
animal. Such is, indeed, found to be the case, until at last, in the
lowest vertebrates, such as the lamprey, and in forms like Amphioxus,
the notochord persists throughout the life of the animal as a large
important axial supporting rod.

This rod has a number of striking characteristics which distinguish
it from all other structures, and are the only means of guessing its
probable origin. Its position in the body is always the same in all
vertebrates and is very significant, for it lies just ventrally to the
central nervous system, along nearly the whole length of the animal,
not quite the whole length, for it invariably terminates close to the
place where the infundibulum comes to the surface of the brain ; it
is, in fact, always confined to the infra-infundibular and spinal cord
part of the central nervous system. Interpreting this into the
language of the arthropod, it means that a rod was formed just
ventrally to the nervous system, which extended the whole length
of the infracesophageal and ventral chain of ganglia, and terminated
at the orifice of the mouth. Moreover, this rod was unsegmented,
for the notochord is devoid of segmentation.

At the anterior end the rod tapers to a point, as in Fig. 166.
In its middle part it is very large and conspicuous, cylindrical in
shape ; its interior is filled with a peculiar vacuolated tissue, different
to any other known vertebrate tissue, which has therefore received
the name of notochordal tissue. Outside this is a thick sheath
formed of many layers, of which the external one gives the staining
reactions of elastin, and is called the external elastic layer. Between
this sheath and the notochordal tissue a thin layer of lining cells, of
normal appearance, is conspicuous in Ammocoetes. These cells secrete
the layers of the sheath, and have originally, by proliferation, given
rise to the notochordal tissue. In the notochord of Ammoccetes
there is no sign of either nerves, blood-vessels, or muscles.

The centre of the notochord presents the appearance of a slight
slit, as though it had originated from a tube, and that is the opinion
now generally held, for its mode of formation in the embryo is as that
of a tube formed from an open groove, as will be explained immediately.

We may, then, conceive of the notochord as originally a tube lying
in the mid-line just ventrally to the central nervous system, and ex-
tending from the original mouth to the end of the body. Translate
this into the language of the arthropod and it denotes a tube on the

436

THE ORIGIN OF VERTEBRATES

mid- ventral surface of the body, which extended from mouth to anus.
Such a tube might be formed from the mid- ventral surface as follows : —
In Fig. 163, A, the lining of the ventral surface between, two
appendages is represented flat, in B is shown how the formation of a
solid rod may arise from the bulging of that ventral surface, and
in C how a groove on that surface may lead to the formation of a
tube between the two appendages. The difference between a noto-
chordal rod formed as in B from that in C would be shown in the
sheath, for in B the sheath would be formed from the cuticle of the
lining cells, and in C from the basement membrane. The structure
of the sheath is in accordance with the embryological evidence that
the notochord is formed as a tube from a groove, as in C, and not as
a solid rod as in B, for it possesses a well-marked elastin layer, and
elastin has never yet been found as a constituent of any cuticular
secretion, but invariably in connection with basement-membranes.

A J3 C

Fig. 1G3.— Diagram of two possible methods of the Formation of a Notochord.

The position, then, of the notochord and its method of formation
suggests that the mid-ventral surface of the arthropod ancestor of the
vertebrate formed a deep groove between the bases of all the proso-
matic, mesosomatic, and metasomatic appendages, which was sub-
sequently converted into a tube extending along the whole of the
body between mouth and anus, and finally, by the proliferation of its
lining cells and their conversion into notochordal tissue, became the
notochordal rod of the vertebrate.

As already frequently stated, Apus and Branchipus are the two
living arthropods which most nearly resemble the extinct trilobites.
The beautiful specimens of Triartbrus (Fig. 165) found by Beecher
sjive an idea of the under surface of the trilobite such as has never
been obtained before, and demonstrate how closely the condition of
things found in Apus (Fig. 164) was similar to that occurring in the
trilobites. In both cases the mid-ventral surface of the animal
formed a deep groove which extended the whole length of the

THE NOTOCHORD AND ALIMENTARY CANAL 437

animal ; on each side of this groove in Apus are closely set the
gnatho-bases of the appendages, in such a manner that the groove
can be easily converted into a canal by the movements of these bases
— a canal which, owing to the great number of the appendages and
their closeness to each other, can be completely and efficiently closed.
All those who have seen Apus in the living state assert that this
canal so formed is actually used by the animal for feeding purposes.
By the movements of the gnatho-bases food is passed up from the

Fig. 164. — Undee-Subface op Apus.
(After Beonn.)

Fig. 165. — Undeb - Sueface of a
Teilobite (Triarthrus). (From
Beeches.)

hind end of the animal along the whole length of this ventral canal
to the mouth, where it is taken in and swallowed. In this way Apus
has been seen to swallow its own eggs.

In the trilobites there is a similar deep channel formed by the
mid-ventral surface, similar gnatho-bases, and closely set appendages,
and the membrane of this ventral groove was extremely thin.

Here, then, in the very group of animals which were the pro-
genitors of the presumed palaeostracan ancestor of the vertebrate — a
group which is characterized by its extensive prevalence and its

438 THE ORIGIN OF VERTEBRATES

enormous variety of form during the great trilobite era — the forma-
tion of a mid-ventral canal out of this deep ventral groove is seen to
be not only easy to imagine, but most probable, provided that a
necessity arose for such a conversion.

For what purpose might such a tube have been formed ? I would
suggest that it might have acted as an accessory food-channel, which
was of sufficient value at the time to give some advantage in the
struggle for existence to those members of the group who were thus
able to supplement their intake of food, but at the same time was
so inefficient that it was quickly superseded by the new alimentary
canal, and thus losing its temporary function, became solid, and was
utilized to form an axial supporting rod.

There is a very considerable amount of evidence in favour of the
view that the notochord was originally a digestive tube ; in fact, as
far as I know, this conclusion is universally accepted. The evidence
is based essentially upon its development and upon its structure. It
is formed in the vertebrate from the same layer as the alimentary
canal, i.e. the hypoblast, and in Amphioxus it commences as a
groove in the dorsal wall of the future alimentary canal ; this groove
then closes to form the tube of the notochord, and separates from
the alimentary canal. Embryologically, then, the notochord is
looked upon as a tube formed directly from the alimentary canal.

As regards its structure, its tissue is, as already stated, something
sui generis. Notochordal tissue lias no resemblance to bone or
cartilage, or any of the usual supporting tissues. Such a tissue is
not, however, entirely confined to the notochord of the vertebrates,
but tissue closely resembling it has been found not only in Amphioxus
and the Tunicata, but in certain other invertebrates, in the Entero-
pneusta (Balanoglossus, etc.), in Cephalodiscus, and in Actinotro-
cha. In all these latter cases, such a tissue is invariably found in
disused portions of the alimentary canal ; a diverticulum of the
alimentary canal becomes closed, vacuolation of its lining cells takes
place, and a tissue resembling notochordal tissue is formed.

Owing to the notochord being invariably so striking and mys-
terious a feature of the lowest vertebrates, the term vertebrate, which
is inappropriate in the members of the group which do not yet possess
vertebras, has been largely superseded by the term chordate, with the
result of attributing an undue preponderance to this tissue in any
system of classification. Hence, wherever any animal has been found

THE NOTOCHORD AND ALIMENTARY CANAL 439

with a tissue resembling that of the notochord, enthusiasts have
immediately jumped to the conclusion that a relationship must exist
between it and the chordate animals ; and, accordingly, they have
classified such animals as follows : Amphioxus belongs to the
group Cephalochorda because the notochord projects beyond the
central nervous system ; the Tunicata are called Urochorda because
it is confined to the tail ; the Enteropneusta, Hemichorda, because
this tissue is confined to a small diverticulum of the gut, and,
finally, Diplochorda has been suggested for Actinotrocha and Pho-
ronis because two separate portions of the gut are transformed
in this way.

This exaggerated importance given to any tissue resembling in
structure that of the notochord is believed in by many of those who
profess to be our teachers on this subject, the very men who can
deliberately shut their eyes to the plain reading of the story of the
pineal eyes, and say, " In our opinion this pineal organ was not an
eye at all."

The only legitimate inference to be drawn from the similarity of
structure between the notochord and these degenerated gut-diverti-
cula, is that the structure of the notochord may have arisen in the
same way, and that therefore the notochord may once have func-
tioned as a gut. With cessation of its function its cells became
vacuolated, as in these other cases, and its lumen became filled with
notochordal tissue. This evidence strongly confirms the suggestion
that the notochord was once a digestive tube, but by no means
signifies that such tissue, wherever found, indicates the presence
of a notochord.

In order to resemble a notochord, this tissue must possess not
only a definite structure but a definite position, and this position is a
remarkably striking and suggestive one. The notochordal tube is
unsegmented, although the vertebrate is markedly segmented. But
in all segmented animals the only unsegmented tube which extends
the whole length of the body, from mouth to anus, is invariably
the gut. In the vertebrate there are three such tubes : (1) the gut
itself, (2) the central canal of the nervous system, and (3) the
notochordal tube.

The first is the present gut, the second the gut of the invertebrate
ancestor, and the third the tube in question.

These three unsegmented tubes, extending along the whole length

44Q

THE ORIGIN OF VERTEBRATES

By.

of the segmented animal, constitute the great peculiarity of the
vertebrate group ; it is not the unsegmented notochord alone which
requires explanation, but the presence of three such tubes in the
same animal. Any one of them might be the unsegmented gut of
the segmented animal. The most ventral tube is the actual gut of
the present vertebrate ; the most dorsal — the neural canal — was,
according to my view, the original gut of the invertebrate ancestor ;
the middle one — the notochordal tube — was, in all probability, also
once a gut, formed at the time when the exigencies of the situation
made it difficult for food to pass along the original gut.

Yet another circumstance in favour of this suggestion is the very

^^^..:,,!-"^.^j,i:.iiiiiii,.ii. mmmmmmmtmtm

;t-.v;--^-^.\\r:.V.'.<^:::.v;.-.^r/vf^iiy.-.v.^-.-.-'.a^-^,|-^:/|'.-^;.;^l

a .•■■iina».-.v.--:..-,--.-.---.'-rf.w rr'^-^^;nrfi7Wrr

' .'» i in..

^.n..-^.-..M.V,.....V...,...l..^ta..vv.-.....vV-.:._ UjiV.vj^^:;

Fig. 166. — Diagram to show the Meeting of the Four Tubes in such a

Vertebrate as the Lamprey.

Nc, neural canal with its infundibular termination ; Nch., notochord ; Al., alimentary
canal with its anterior diverticulum ; Hi/., hypophysial or nasal tube; Or., oral
chamber closed by septum.

striking position of the anterior termination of the notochord. It
terminates at the point of convergence of three structures : —

(1) The tube of the hypophysis or nasal tube.

(2) The infundibulum or old mouth-termination.

(3) The notochordal tube.

To these may be added, according to Kupffer, in the embryonic
stage, the anterior diverticulum of the gut (Fig. 16B).

This is a very significant point. Here originally, in the inverte-
brate stage, the olfactory passage opened into the old mouth and
oesophagus. Here, finally, in the completed vertebrate the same
olfactory passage opens into the new pharynx. In the stage between
the two it may well have opened into an intermediate gut, the noto-
chordal tube, its separation from which would leave the end of the

THE NOTOCHORD AND ALIMENTARY CANAL 44 1

notochord blind, just as it had already left the end of the infundi-
bulum blind.

The whole evidence points to the derivation of the notochord
from a ventral groove on the surface of the animal, which closed
to form a tube capable of acting as an accessory gat at the critical
period before the new gut was fully formed. The essentials of a gut
tube are absorption and digestion of food ; is it likely that a tube
formed as I have suggested would be efficient for such purposes ?

As far as absorption is concerned, no difficulty would arise.
The gut of the arthropod is lined with a thin layer of chitin,
which is traversed, like all other chitinous surfaces, by fine canali-
culi. Through these canaliculi, absorption of fluid material takes
place, from the gut to the body. Similar canaliculi occur in the
chitin covering the animal externally, so that, if such external
surface formed a tube, and food in the right condition for absorption
passed along it, absorption could easily take place through the
chitinous surface. The evidence of Apus proves that food does
pass along such a tube in the open condition, and in the trilobites
the chitinous surface lining a similar groove was apparently very thin,
a condition still more favourable to such an absorption process.

At first sight the second essential of a gut -tube — the power
of digestion — appears to present an insuperable difficulty to this
method of forming an accessory gut-tube, for it necessitates the for-
mation of a secretion capable of digesting proteid material by the
external cells of the body, whereas until recently it was supposed
that such a function was confined to cells belonging to the so-called
hypoblastic layer. Experiments were made now years ago of
turning a Hydra inside out so that its internal layer should become
external, and vice versa, and they were said to have been successful.
Such an animal could go on living and absorbing and digesting food,
although its epiblastic surface was now its digestive internal surface.
More recent observations have shown that these experiments were
fallacious. At night-time, when the observer was not looking, the
hydra rein verted itself, so that again its original digestive surface
was inside and it lived and prospered as before.

Another piece of evidence of somewhat similar kind, which has
not as yet been discredited, is seen in the Tunicata. In many of
these, new individuals are formed from the parent by a process of
budding, and it has been proved that frequently the gut of the new

442 THE ORIGIN OF VERTEBRATES

individual thus budded off arises not from the gut or hypoblastic
layer of the parent, but from the surface or epiblastic layer. Such
gut so formed possesses as efficient digestive powers as the gut of
the parent.

The most remarkable evidence of all has been afforded by
Miss Alcock's experiments. She examined the different tissues of
Ammoccetes for the express purpose of finding out their power of
digesting fibrin, with the result that the most active cells were
those of the liver. Next in activity came the extract of the lining
cells of the respiratory chamber and of the skin. The intestine
itself when freed from the liver-secretion had very little digestive
power; extracts of muscle, nervous system, and thyroid gland had
no power whatever, but the extract of the skin-cells possessed a
powerful digesting action.

Furthermore, it is not necessary to make an extract of the
skin in order to obtain this digestive fluid, for under the influence
of chloroform the skin of Ammoccetes secretes copiously, and this
fluid thus secreted was found to possess strong digestive powers. So,
also, Miss Alcock has demonstrated the power of digesting fibrin
in a similar secretion of the epithelial cells lining the carapace of
the crayfish. In both cases a very plausible reason for the presence
of a digestive ferment in a skin-secretion is found in the necessity
of preventing the growth of parasites, fungoid, or otherwise, especially
in those parts where the animal cannot keep itself clean by
' preening.' Thus in a crayfish, in which the oesophageal commissures
had been cut, fungus was found to grow on the ventral side, but not
on the dorsal carapace. The animal was accustomed to keep its
ventral surface clean by preening ; owing to the paralysis it could
not do so, and consequently the fungus grew there. In the lamprey
I found that wherever there was a removal of the surface-epithelium,
from whatever cause, that spot was immediately covered with a
fungoid growth, although in the intact lamprey the skin was
invariably smooth and clean.

I imagine, then, that this digestive power of the skin arose as
a protective mechanism against parasitic attacks ; it is self-evident
how a tube formed of such material must ab initio act as a digestive
tube.

In yet another respect this skin secretion of Ammoccetes is most
instructive. The surface of Ammoccetes is absolutely smooth, no scales

THE NOTOCHORD AND ALIMENTARY CANAL 443

of any kind exist ; this smoothness is due to the presence of a very
well-defined cuticular layer secreted by the underlying epithelial cells.
This cuticle is very much thicker than is usually found in vertebrates,
and, strangely enough, has been thought to contain chitin. Whether
it really contains chitin or not I am unable to say, but it certainly
resembles a chitinous layer in one respect ; it is perforated by innu-
merable very fine tubes or canaliculi, along which, by appropriate
staining, it is easy to see the secretion of the underlying cell pass
to the exterior (Fig. 140). This marked digestive power of the skin
of Ammocoetes, together with the easy passage of the secretion
through the thin cuticular layer, renders it almost certain that a tube
formed from the deep ventral groove of the trilobite would, from the
very first, act as a digestive as well as an absorbent tube ; in other
words, the notochord as soon as formed was able to act as an accessory
digestive tube.

This suggested origin of the notochord from a groove along the mid-
ventral surface of the body not only indicates a starting-point from a
markedly segmented portion of the body, but also points to its forma-
tion at a stage previous to the formation of the operculum by the
fusion of the two foremost mesosomatic appendages — indicates there-
fore its formation at a stage more nearly allied to the trilobite than to
the sea-scorpion. The chance of ever finding any direct evidence of
such a chordate trilobite stage appears to me exceedingly improbable,
and I greatly fear that this conception of the mode of formation of
the notochord can never be put to direct proof, but must always
remain guesswork.

On the other hand, evidence of a kind in favour of its origin from
a segmented part of the body does exist, and that evidence has this
special value, that it is found only in that most primitive animal,
Amphioxus.

This evidence is as follows : —

At fairly regular intervals, the sheath of the notochord is inter-
rupted on each side of the mid-dorsal line by a series of holes, which
penetrate the whole thickness of the sheath. This dorsal part is
pressed closely against the spinal cord, and through these holes fibres
appear to pass from the spinal cord to the interior of the notochord.
So greatly do these fibres present the appearance of ventral roots to
the notochord, that Miss Piatt looks upon them as paired motor roots
to the notochord, or at all events as once having been such motor

444 THE ORIGIN OF VERTEBRATES

roots. Lwoff and Eolph both describe a direct communication
between the spinal cord and the notochord by means of fibres
passing through these holes, without however looking upon this con-
nection as a nervous one. Joseph alone asserts that no absolute
connection exists, for the internal elastic layer of the notochord,
according to him, is not interrupted at these holes, and forms, therefore,
a barrier between the fibres from the spinal cord and those from the
interior of the notochord. Still, whatever is the ultimate verdict as
to these fibres, the suggestive fact remains of the spaces in the
notochordal sheath and of the corresponding projecting root-like fibres
from the spinal cord. The whole appearance gives the impression of
some former connection, or rather series of connections, between the
spinal cord and the notochord, such as would have occurred if nerves
had once passed into the notochord. On the other hand, such nerves
were not arranged segmentally with the myotomes, for, according to
Joseph, in the middle of the animal ten to twelve such holes occur in
one body-segment. In Apus the appendages are more numerous than
the body-segments, so that it is not necessary for a segmental arrauge-
ment to coincide with that of the body-segments.

The Origin of the Alimentary Canal.

In close connection with the notochord is the alimentary canal.
Any explanation of the one must be of assistance in explaining
the other.

According to the prevalent embryological teaching, the body is
formed of three layers, epiblast, hypoblast, and mesoblast, and the
gastrtea theory of the origin of all Metazoa implies of necessity that
the formation of every individual commences with the formation of
the gut. For this reason the alimentary canal must in every case
be regarded as the earliest formed organ, however late in the develop-
ment it may attain its finished appearance. Hence the notochord is
spoken of as developed from the mid-dorsal wall of the alimentary
canal. It is possible to look at the question the other way round,
and suppose that the organ whose development is finished first is
older than the one still in process of making. In this case it would
be more right to say a ventral extension of the tissue, which gives
rise to the notochord, takes place and forms the alimentary canal.
It is, to my mind, perfectly possible, and indeed probable, that

THE NOTOCHORD AND ALIMENTARY CANAL 445

the formation of the vertebrate alimentary canal was a repetition of
the same process which had already led to the formation of the
notochordal tube. The formation of the anterior part of the ali-
mentary canal in Ammoccetes at the time of transformation strongly
suggests the marked similarity of the two processes.

Of all the startling surprises which occur at transformation, this
formation of a new anterior gut is the most startling. From the
oral chamber of Petromyzon two tubes start : the one leads into the
gill-chambers, is known as the bronchus, and is entirely concerned
with respiration ; the other leads without a break from the mouth
to the anus, has no connection with respiration, and is the alimentary
canal of the animal. Any one looking at Petromyzon would say
that its alimentary canal was absolutely non-respiratory in character.
Before transformation, this kind of alimentary canal commences at
the end of the respiratory chamber ; from here to the anus it is of
the same character as in Petromyzon, but in Ammoccetes the non-
respiratory anterior part simply does not exist : the whole anterior
chamber is both respiratory and affords passage to food. This part
of the alimentary canal of the adult is formed anew. We see, then,
here the formation of a part of the alimentary canal taking place, not
in an embryo full of yolk, but in a free-living, independent, grown-up
larval form in which all yolk has long since disappeared : a condition
absolutely unique in the vertebrate kingdom, but one which more
than any other may be expected to give a clue to the method of
formation of a vertebrate gut.

The formation of this new gut can be easily followed at trans-
formation, and was originally described by Schneider. His statement
has been confirmed by Nestler, and its absolute truth has been
demonstrated to me again and again by Miss Alcock, in her specimens
illustrative of the transformation process. First, in the mid-dorsal
line of the respiratory chamber a distinct groove is formed, the
edges of which come together and form a solid rod. This solid rod
blocks the opening of the respiratory chamber into the mid-gut, so
that during this period of the transformation no food can pass out of
the pharyngeal chamber. A lumen then begins to appear in this
solid rod at the posterior end, which steadily advances mouthwards
until it opens into the oral chamber and thus forms an open tube
connecting the mouth with the gut.

Here, then, is the foundation of a new gut on very similar lines

446 THE ORIGIN OF VERTEBRATES

to that of the notochord, by the conversion of a groove into a tube.
Still more suggestive is it to find that the tube so formed has no
appearance whatever of segmentation ; it is as unsegmented as the
rest of the gut, although, as is seen in Fig. 62, the dorsal wall of
the respiratory chamber from which it arose is as markedly seg-
mented as any part of the animal. Here under our very eyes, in the
course of a few days or weeks, an object-lesson in the process of the
manufacture of an alimentary canal is carried out and completed,
and the teaching of that lesson is that a gut-tube may be formed
in the same way as the notochordal tube, by the conversion of a
grooved surface into a canal, and that gut-tube so formed, like the
notochord, loses all sign of segmentation, even although the original
grooved surface was markedly segmented.

The suggestion then is, that the new gut may have been formed
by a repetition of the same process which had already given origin
to the notochord.

Such a method of formation is not, in my opinion, opposed to the
evidence given by embryology, but in accordance with it ; the dis-
cussion of this point will come best in the next chapter, which treats
of the embryological evidence as a whole, and will therefore be left
till then.

The Evidence given by the Innervation of the Vertebrate

Alimentary Canal.

Throughout this investigation the one fixed landmark to which all
other comparisons must be referred, is the central nervous system, and
the innervation of every organ has given the clue to the meaning of
that organ. So also it must be with the new alimentary canal ; by its
innervation we ought to obtain some insight into the manner of its
origination. In any organ the nerves which are specially of value in
determining its innervation, are of necessity the efferent or motor
nerves, for the limits of their distribution in the organ are much
more easily determined than those of the afferent or sensory nerves.
The question therefore of primary importance in endeavouring to
determine the nature of the origin of the alimentary canal from its
innervation is the determination of the efferent supply to the
musculature of its walls.

Already in previous chapters a commencement has been made in

THE NOTOCHORD AND ALIMENTARY CANAL 447

this direction ; thus the musculature of the oral chamber has been
derived directly from the musculature of the prosomatic appendages ;
the muscles "which move the eyes from the prosomatic and rneso-
somatic dorso- ventral somatic muscles ; the longitudinal body-muscles
from the dorsal longitudinal somatic muscles of the arthropod ; the
muscles of respiration from the dorso-ventral muscles of the meso-
somatic appendages.

In all these cases we have been dealing with striated musculature
and consequently with only the motor nerves of the muscle ; but the
gut posterior to the pharyngeal or respiratory chamber contains
unstriped instead of striped muscle, and is innervated by two sets of
nerves, those which cause contraction and are motor, and those which
cause relaxation and are inhibitory. It is by no means certain that
these two sets of nerves possess equal value from a morphological
point of view. The meaning of an inhibitory nerve is at present
difficult to understand, and in this instance, is rendered still more
doubtful owing to the presence of Auerbach's plexus along the whole
length of the intestine — an elaborate system of nerve-cells and nerve-
fibres situated between the layers of longitudinal and circular muscles
surrounding the gut-walls, which has been shown by the recent
experiments of Magnus, to constitute a special enteric nervous system.
One of the strangest facts known about the system of inhibitory
nerves is their marked tendency to leave the central nervous system
at a different level to the corresponding motor nerves, as is well
known in the case of the heart, where the inhibitory nerve — the
vagus — arises from the medulla oblongata, while the motor nerve — the
augmentor or accelerator — leaves the spinal cord in the upper thoracic
region. It is very difficult to obtain any idea of the origin of such a
peculiarity ; I know of only one suggestive fact, which concerns the
innervation of the muscles which open and close the chela of the
crayfish, lobster, etc. These muscles are antagonistic to each other,
and both possess inhibitory as well as motor nerves. The central
nervous system arrangements are of such a character that the contrac-
tion of the one muscle is accompanied by the inhibition of its opposer,
and the nerves which inhibit the contraction of the one, leave the
central nervous system with the nerves which cause the other to
contract. Thus the inhibitory and motor nerves of either the abduc-
tor (opener) or adductor (closer) muscles of the crayfish claw do not
leave the central nervous system together, but in separate nerves.

448 THE ORIGIN OF VERTEBRATES

If now for some cause the one set of muscles either disappeared,
or were so altered as no longer to present any appearance of
antagonism, then there would' be left a single set of muscles, the
inhibitory and motor nerves of which would leave the central
nervous system at different levels, and the older such systems might
be, the greater would be the modification in the shape and arrange-
ments of parts in the animal, so that the two sets of fibres might
ultimately arise from very different levels.

As mentioned in the introductory chapter, the whole of this
investigation into the origin of vertebrates arose from my work on
the system of efferent nerves which innervate the vascular and
visceral systems. One of the main points of that investigation
was the proof that such nerves did not leave the central nervous
system uniformly along the whole length of it, but in three great
outflows, cranial, thoracico-lumbar, and sacral ; there being two
marked gaps separating the three outflows, caused by the inter-
polation of the plexuses for the innervation of the anterior and
posterior limbs respectively. All these nerves are characterized by
the presence of ganglion-cells in their course to the periphery, they
are, therefore, distinguished from ordinary motor nerves to striated
muscle in that their impulses pass through a ganglion-cell before
they reach the muscle.

The ganglia of the large middle thoracico-lumbar outflow
constitute the ganglia of the sympathetic system.

The functions of the nerves constituting these three outflows are
very different, as I pointed out in my original papers. Since then a
large amount of further information has been obtained by various
observers, especially Langley and Anderson, which enable the
following statements to be made : —

All the nerves which cause contraction of the unstriped muscles
of the skin, whether pilomotor or not, all the nerves which cause
secretion of sweat glands wherever situated, all the nerves which
cause contraction or augmentation of the action of muscles belonging
to the vascular system, all the nerves which are motor to the muscles
belonging to all organs derived from the Wolffian and Miillerian
ducts, e.g. the uterus, ureters, urethra, arise from the thoracico-
lumbar outflow, never from the cranial or sacral outflows. It is
essentially an efferent skin-system.

On the other hand, the latter two sets of nerves are concerned

THE NOTOCHORD AND ALIMENTARY CANAL 449

with the supply of motor nerves to the alimentary canal ; they form
essentially an efferent gut-system in contradistinction to the sympa-
thetic or skin-system.

A marked distinction exists between these cranial and sacral
nerves. The vagus never supplies the large intestine, the sacral
nerves never supply the small intestine. Associated with the large
intestine is the bladder, the whole system arising from the original
cloacal region ; the vagus never supplies the bladder, its motor
nerves belong to the sacral outflow. The motor nerves to the
ureters, to the urethra, and to the trigonal portion of the bladder
between the ureters and the urethra, do not arise from the sacral
outflow, but from the thoraeico-lumbar. These muscles belong really
to the muscles in connection with the Miillerian and "Wolffian ducts
and skin, not to the cloacal region.

The motor innervation then of the alimentary caual reveals this
striking and suggestive state of affairs. The motor innervation of

O CO

the whole of the small intestine arises from the cranial region, and
is immediately followed by an innervation from the sacral region for
the whole of the muscles of the cloaca. It thus indicates a head-
region and a tail-region in close contiguity, the whole of the spinal
cord region between these two extremes being apparently unrepre-
sented. Xot, however, quite unrepresented, for Elliott has shown
recently that the ileo-colic valve at the junction of the small and
large intestine is in reality an ileo-colic sphincter muscle, and that
this muscle receives its motor nerves neither from the vagus nor
from the sacral nerves, but from the thoraeico-lumbar outflow or
sympathetic system. This may mean one of two things, either that
a band of fibres belonging to the skin-system has been added to the
gut-musculature, for the purpose of forming a sphincter at this spot,
or that the region between the vagus territory and the cloaca is repre-
sented by this small band of muscle. The second explanation seems
to me the more probable of the two. Between the mesosomatic
region represented by the vagus, and the cloacal region, there existed
a small metasomatic region, represented by the pronephros, with its
segmental duct, as already discussed in Chapter XII. That part of
the new alimentary canal which belonged to this region is the short
piece indicated by the ileo-colic sphincter, and innervated, therefore,
from the same region as the organs derived from the segmental duct.
Such innervation seems to me to suggest that originally the

2 G '

450 THE ORIGIN OF VERTEBRATES

vertebrate consisted, as far as its gut was concerned, of a prosomatic
and mesosomatic (branchial) region, close behind which came the
cloaca and anus. Between the two there was a short metasomatic
region (possibly pronephric), so that the respiratory chamber did not
open directly into the cloaca.

Such an interpretation is, I think, borne out by the study of the
most ancient forms of fish. In Bothriolepis, according to Patten,
and in Drepanaspis, according to Traquair, the cloacal region and
anus follow immediately upon the posterior end of the head-shield,
i.e. immediately after that region which presumably contained the
branchia3. Similarly, on the invertebrate side, all those forms which
resembled Limulus must have possessed a very short region between
the branchial and cloacal parts of the body. The original cloacal
part of the vertebrate gut may well have been the original cloaca
of the arthropod, into which its intestine emptied itself, especially
when we see the tendency of the scorpion group of animals to
form an accessory cloacal pouch known as the stercoral pouch or
pocket.

Again, it is striking to see how, in certain of the scorpion group,
e.g. Thelyphonus and Phrynus, there is a caudal massing of the
central nerve-cells as well as a cephalic massing, so that their
central nervous system is composed of a cephalic and caudal brain.
These two brains are connected together by commissures extending
the whole length of the body, in which I have been unable to find
any sign of ganglion-cells. What this caudal brain innervates I
do not know ; it is, I think, a matter worth further investigation,
especially as there are many indications in the vertebrate that the
lumbo-sacral region of the cord possesses higher functions than the
thoracic region.

The method of formation of the alimentary canal as indicated by
its innervation is as follows : —

In front an oral chamber, formed, as already pointed out, by
the modification of the prosomatic appendages, followed by a
respiratory chamber, the muscles and branchise of which were
the muscles and branchise of the mesosomatic appendages. This
mesosomatic, or branchial, part was in close contiguity to the cloaca
and anus, being separated from it only by a short tube formed in the
metasomatic or pronephric region.

I imagine that this connection was originally in the form of an

THE NOTOCHORD AND ALIMENTARY CANAL 45 1

open groove, as already explained for both notochord and the
anterior part of the gut itself in Ammoccetes ; an open groove
formed from the mid-ventral surface of the body, on each side of
which were the remnants of the pronephric appendages. By the
closure of this groove ventrally, and the growing round of the pleural
folds, as already suggested, the remains of the pronephric appendages
are indicated by the segmental duct and the form of the vertebrate
body is attained.

Even in the branchial region the same kind of tiring must, I
think, have occurred. The grooved ventral surface became a tube, on
each side of which were lying in regular order the in-sunk branchial
appendages, the whole being subsequently covered by the pleural
folds to form an atrial chamber. A tube thus formed from the
grooved ventral surface would carry with it to the new ventral
surface the longitudinal venous sinuses, and thus form, in the way
already suggested, the heart and ventral aorta. Posterior to the
heart in the pronephric region, the same process would give rise to
the sub- intestinal vein.

The evidence of comparative anatomy bears out most con-
clusively the suggestion that in the original vertebrate the gut was
mainly a respiratory chamber. In man and all mammals the oral
chamber opens into a small pharynx, followed by the oesophagus,
stomach and small intestine. Of this whole length, a very small
part is taken up by the pharynx, in which, in the embryo, the
branchial arches are found, showing that this represents the original
respiratory part of the gut. In the ordinary fish this branchial part
is much more conspicuous, occupies a large proportion of the gut,
and in the lowest fishes, such as Ammoccetes and Amphioxus, the
branchial region extends over a large portion of the animal, while
the intestine proper is a straight tube, the length of which is in-
significant in comparison with its length in the higher vertebrates.

Such a tube was able to act as a digestive tube, owing, as already
pointed out, to the digestive powers of the skin- epithelium, and I
imagine at first the respiratory chamber, seeing that it composed
very nearly the whole of the gut, was at the same time the main
digestive # chamber ; even in Ammoccetes its digestive power is
superior to that of the intestine itself.

Just posterior to the branchial part a diverticulum of the gut was
formed at an early stage, as seen in Amphioxus, and provided the

452 THE ORIGIN OF VERTEBRATES

commencement of the liver. This simple liver-diverticulum became
the tubular liver of Ammoccetes, and formed, curiously enough, not
a glandular organ of the same character as the liver of the higher
vertebrates, but a hepato-pancreas, like the so-called liver of the
arthropods, which also is a special diverticulum of the gut, or rather
the main true gut of the animal. In both cases the liver is the chief
agent in digestion, for in Ammoccetes the liver-extract is very much
more powerful in the digestion of proteids than the extract of any
other organ tried by Miss Alcock. Subsequently in the vertebrate
the gastric and pancreatic glands arise and relieve the liver of the
burden of proteid digestion.

It is, to my mind, somewhat significant that the liver on its first
formation in the vertebrate should have arisen as a digestive organ of
the same character as the so-called liver in the arthropods ; whether
it originally belonged to any separate segment is in our present state
of knowledge difficult to say.

Conclusion.

In conclusion, I will endeavour to illustrate crudely the way in
which, on my theory, the notochord and vertebrate gut may have
been formed, the agencies at work being in the main two, viz. the
dwindling of appendages as mere organs of locomotion, and the
conversion of a ventral groove into a tube.

I imagine that, among the Protostraca, forms were found some-
what resembling trilobites with markedly polycbietan affinities ;
which, like Apus, possessed a deep ventral groove from one end of
the body to the other, and also pleural fringes, as in many trilobites.
This might be called the Trilobite stage (Fig. 167, A).

This groove became converted into a tube and so gave rise to the
notochord, while the appendages were still free and the pleura' had
not met to form a new ventral surface. This might be called the
Chordate Trilobite stage (Fig. 167, B).

Then, passing from the protostracan to the paheostracan stage,
the oral and respiratory chambers were formed, not communicating
with each other, in the manner described in previous chapters, a
ventral groove in the metasomatic region being the only connection
between respiratory chamber and cloaca. This might be called the
Chordate Pakeostracan stage (Fig. 167, C).

THE NOTOCHORD AND ALIMENTARY CANAL 453

N. N. Nc.

N. Nc.

Fig. 167. — A, Diagram of Section through a Trilobite-like Animal; B,
Diagram to illustrate the Suggested Formation of the Notochord
from a Ventral Groove ; C, Diagram to illustrate the Suggested
Formation of the Post-Branchial Gut by the continuation of the same
process of Ventral Groove-Formation, combined with Obliteration of
Appendages and Growth of Pleural Folds ; D, Diagram to illustrate
the Completion of the Vertebrate Type by the Meeting of the Pleural
Folds in the Mid-Ventral Line with the Obliteration of the Atrial
Cavity and the Conversion of the Ventral Groove into the closed
Alimentary Canal.

.4/., alimentary cannl; N., nervous system; My., myotome; PL, pleuron ; App.,
appendage; Kcph., nephrocele; Met., metaccele ; Sd., segmental duct; Mcs.,
mesonephros ; At., atrial chamber; Nc, notcehord ; H., heart; F., fat body;
Ng., notochordal groove. (These diagrams are intended to complete the
diagrams on p. 413, which, as stated there, were purposely left incomplete.)

454 THE ORIGIN OF VERTEBRATES

Finally, with the conversion of this groove into a tube, the opening
of the oral into the respiratory chamber, and the formation of an
atrium by the ventralwards growth of the pleural folds, the formation
of a Vertebrate was completed (Fig. 167, D).

In my own mind I picture to myself an animal which possessed
eurypterid and trilobite characters combined, in which a notochordal
tube had been formed in the way suggested, and a respiratory chamber
which communicated with the cloaca by means of a grooved channel
along the mid-ventral line of the metasomatic portion of the body.
On each side of this channel were the remains of the metasomatic
appendages (pronephric). The whole was enveloped in the pleural
folds, which probably at this time did not yet meet in the middle
line to form a new ventral surface. This respiratory chamber, owing
to the digestive power of the epidermis, assisted, in the process of
alimentation to such an extent as to supersede the temporary noto-
chordal tube, with the effect of bringing about the conversion of the
metasomatic groove into a closed canal, and so the formation of an
alimentary tube continuous with the respiratory chamber. The
amalgamation of the pleural folds ventrally completed the process,
and so formed an animal resembling the Cephalaspidse, Ammoccetes,
or Amphioxus.

I have endeavoured in this chapter to make some suggestions
upon the origin of the notochord and of the vertebrate gut in accordance
with my theory of the origin of vertebrates. I feel, however, strongly
that these suggestions are much more speculative than those put
forward in the previous chapters, and of necessity cannot give the
same feeling of soundness as those based directly upon comparative
anatomy and histology. Still, the fact remains that the origin of the
notochord is at present absolutely unknown, and that my speculation
that it may have originated as an accessory digestive tube is at all
events in accordance with the most widely spread opinion that it
arises in close connection with an alimentary canal.

CHAPTER XIV

THE PRINCIPLES OF EMBRYOLOGY

The law of recapitulation. — Vindication of this law by the theory advanced
in this book. — The germ-layer theory. — Its present position. — A physio-
logical not a morphological conception. — New fundamental law required. — ■
Composition of adult body. — Neuro-epithelial syncytium and free-living
cells. — Meaning of the blastula. — Derivation of the Metazoa from the Pro-
tozoa. Importance of the central nervous system for Ontogeny as well as
for Phylogeny. — Derivation of free-living cells from germ-cells. — Meaning
of coelom. — Formation of neural canal. — Gastrula of Amphioxus and of
Lucifer. — Summary.

In a discussion upon this theory of mine, which took place at
Cambridge on November 25 and December 2, 1895, it was said that
such a theory was absolutely and definitely put out of court, because
it contravened the principles of embryology, was opposed, therefore,
to our surest guide in such matters ; and the law was laid down with
great assurance that no claim for genetic relationship between two
groups of animals can be allowed which is based upon topographical
and structural coincidences revealed by the study of the anatomy of
two adult animals, however numerous and striking they may be, if
there are fundamental differences in the embryology of the members
of these two groups.

According to my theory the old gut of the arthropod still exists in
the vertebrate as the tubular lining of the central nervous system,
and the vertebrate has formed a new gut. According to the principles
of embryology as held up to the present, in all animals above the
Protozoa, the different structures of the body arise from three definite
embryonic layers, the epiblast, mesoblast, and hypoblast, and in all
cases the gut arises from the hypoblastic layer. In the vertebrate
the gut also arises from the hypoblast, while the neural canal is
epiblastic. My theory, then, makes the impossible assertion that
what was hypoblast in the arthropod has become epiblast in the
vertebrate, and what was epiblast in the arthropod has become
hypoblast in the vertebrate. Such a conception is supposed to be so

456 THE ORIGIN OF VERTEBRATES

absolutely impossible that it only requires to be stated to be dis-
missed as au absurdity.

Against this opinion I claim boldly that my theory is not only
not contrary to the principles of embryology, but is mainly based
upon the teachings of embryology. I wish here not to be mis-
understood. The great value of the study of embryology for questions
of the sequence of the evolution of animals is to be found in what is
known as the Law of Becapitulation, which asserts that every animal
gives some indication in the stages of its individual development of
its ancestral history. Naturally enough it cannot pass through all
the stages of its past history with equal clearness, for what has taken
millions of years to be evolved has to be compressed into an evolution
lasting only a few months or weeks, or even less.

When in the highest vertebrate a vestigial organ, such as the
pineal gland, can be traced back without leaving the vertebrate
kingdom to a distinct median eye, such as is found in the lamprey,
that rudimentary organ is evidence of an organ which was functional
in the earliest vertebrates or their immediate ancestors. So it is
generally with well denned vestigial organs found in the adult
animal ; they always indicate an organ which was functional in the
near ancestor.

Passing from the adult to the embryo we still find the same law.
Here, also, vestigial organs are met with, which may leave no trace in
the adult, but indicate organs which were functional in the near
ancestor. Thus, but for embryology, we should have no certainty
that the air-breathing vertebrates had been derived from water-
breathing fishes ; the indication is not given by any close resemblance
between the formation of the embryos in their earliest stages, but
by the formation of vestigial gill-arches even in the embryos of the
highest mammal.

For all questions of evolution the presence of vestigial organs in
the embryo is the important consideration, for they give an indication
of near ancestry ; the early formation of the embryo concerns a
much more remote ancestral period, all vestigial organs of which
may well have been lost and obscured by coenogenetic changes. Let
us, then, consider the two things — the vestigial organs and the early
formation of the embryo — separately, and see how far my opponents
are justified in their statement that my theory contravenes the
principles of embryology.

THE PRINCIPLES OF EMBRYOLOGY 457

First, I will take the teachings of vestigial organs and the arrange-
ment of organs found in the vertebrate embryo. Here it is impossible
to say that my theory is contrary to the teaching of embryology, for
as the previous chapters have shown again and again, the argument
is based very largely upon the facts of embryology. In the first
place, the comparison which I have chiefly made is a comparison
between the larval form of a very low vertebrate and the arthropod
group, a comparison which exists only for the larval form, and not
for the adult. The whole theory, then, is based upon a developmental
stage of the vertebrate, and not upon the anatomy of the adult.

Throughout the whole history it seems to me perfectly marvellous
how completely the law of recapitulation is vindicated by my theory
of the origin of the vertebrate. The theory asserts that the clue
to the origin of vertebrates is to be found in the tubular nature
of the central nervous system of the vertebrate ; in that the verte-
brate central nervous system is in reality formed of two things : (1)
a central nervous system of the arthropod type, and (2) an epithelial
tube in the position of the alimentary canal of the arthropod.

Is it possible for embryology to recapitulate such a phylogenetic
history more clearly than is here the case ? In order to avoid all
possibility of our mistaking the clue, the nerve-tube in the embryo
always opens into the anus at its posterior end, while in the larval
Amphioxus it is actually still open to the exterior at the anterior end.
The separateness of the tube from the nervous system at its first
origin is shown especially well in the frog, where, as Assheton has
pointed out, owing to the pigment in the cells of the external layer
of epithelium, a pigmented tube is formed, on the outside of which
the nervous tissue is lying, and step by step the gradual inter-
mingling of the nerve- cells and the pigmented lining cells can be
followed out.

Consider the shape of the nerve-tube when first formed in the
vertebrate. At the cephalic end a simple bulged-out tube with two
simple anterior diverticula, which passes into a narrow straight spinal
tube; from this large cephalic bulging a narrow diverticulum, the
infundibulum, passes to the ventral surface of the forming brain.
This tube is the embryological expression of the simple dilated cephalic
stomach, with its ventral tesophagus and two anterior diverticula,
which opens into the straight intestine of the arthropod. Nay, more,
by its very shape, and the invariable presence of two anterior

458 THE ORIGIN OF VERTEBRATES

diverticula, it points not only to an arthropod ancestry, but to a
descent from a particular group of primitive arthropods. Then
comes the formation of the cerebral vesicles, aud the formation of
the optic cup, telling us as plainly as can be how the invasion of
nervous material over this simple cephalic stomach and its diverticula
has altered the shape of the original tube, and more and more
enclosed it with nervous elements.

So, too, in the spinal cord region. When the tube is first formed,
it is a large tube, the latero-ventral part of which presents two
marked bulgings; connecting these two bulgings is the anterior
commissure. These two lateral bulgings, with their transverse
commissure, represent, with marked fidelity, the ventral ganglion-
masses of the arthropod with their transverse commissure, and occupy
the same position with respect to the spinal tube, as the ganglion-
masses do with respect to the intestine in the arthropod. Then the
further development shows how, by the subsequent growth of the
nervous material, the calibre of the tube is diminished in size, and
the spinal cord is formed.

Again, I say, is it possible to conceive that embryology should
indicate the nature of the origin of the vertebrate nervous system
more clearly than it does ?

It is the same with all the other organs. Take, for instance, the
skeletal tissues. The study of the vertebrate embryo asserts that the
cartilaginous skeleton arose as simple branchial bars and a simple
cranio-facial skeleton, and also that the parenchymatous variety of
cartilage represents the embryonic form. Word for word, the early
embryonic stage of the vertebrate skeleton closely resembles the
stage reached in the arthropod, as shown by Limulus, and again
records, unmistakably, the past history of the vertebrate.

So, too, with the whole of the prosomatic region ; the situation
of the old mouth, the manner in which the nose of the cephalaspidian
fishes arose from the pala30stracan, are all shown with vivid clearness
by Kupffer's investigations of the early stage of Ammocoetes, while
at the same time the closure of the oral cavity by the septum shows
how the oral chamber was originally bounded by the operculum.
Nay, further, the very formation of this chamber embryologically was
brought about by the forward growth of the lower lip, just as it must
have been if the chilaria grew forward to form the metastoma.

So, too, the study of the embryo teaches that the branchiae arise as

THE PRINCIPLES OF EMBRYOLOGY 459

ingrowths, that the heart arises as two longitudinal veins, jnst as the
theory supposes from the facts provided by Limulus and the scorpions.
No indication of the origin of the thyroid gland is given by the study
of its structure in any adult vertebrate, but in the larval form of the
lamprey there is still preserved for us a most graphic record of its
past history.

The close comparisons which it is possible to make between the
eye-muscles of the vertebrate and the recti muscles of the scorpion
group on the one hand, and between the pituitary and coxal glands on
the other, are based upon, or at all events are strikingly confirmed by,
the study of the ccelomic cavities and the origin of these muscles in
the two groups. In fact the embryological evidence of the double
segmentation in the head and the whole nature of the cranial
segments is one of the main foundation-stones on which the whole of
my theory rests.

So it is throughout. Turn to the excretory organs — it is not the
kidney of the adult animal which leads direct to the excretory organs
of the primitive arthropod, but the early embryonic origin of that
kidney.

So far from having put forward a theory which runs counter to
the principles of embryology, I claim to have vindicated the great
Law of Recapitulation which is the foundation-stone of embryological
principles. My theory is largely based upon embryological facts, and
its strength consists in the manner in which it links together into
one harmonious whole, the facts of Embryology, Palaeontology, Ana-
tomy, and Physiology. Why, then, is it possible to assert that my
theory disregards the principles of embryology, when, as we have
seen, embryology is proclaiming as loudly as possible how the verte-
brate arose ? In my opinion, it is because the embryologists have
to a large extent gone wrong in their fundamental principles, and
have attached more weight to these faulty fundamental principles
than to the obvious facts which, looked at thoughtfully, could not
have failed to suggest a doubt as to the correctness of these
' principles.'

The current laws of embryology upon which such weight is laid
are based on the homology of the germinal layers in all Metazoa, and
state that in all cases after segmentation is finished a blastula is
formed, from which there arises a gastrula, formed of an internal
layer, the hypoblast, and an external layer, the epiblast ; subsequently

460 THE ORIGIN OF VERTEBRATES

between these arises a third layer, the mesoblast. These layers are
strictly morphological conceptions, and are stated to be homologous
in all cases, so that the hypoblast of one animal must be homologous
to the hypoblast of another. In order, therefore, to compare two adult
animals for the purpose of finding kinship between them, it is neces-
sary to find whether parts such as the gut, which in both cases have
the same function, arise from the same germinal layer in the embryo.
We can, in fact, have no certainty of kinship, even although the two
animals are built up as far as the adult state is concerned on a
remarkably similar plan, unless we can study their respective
embryos and find out what parts arise from the hypoblast and what
from the epiblast. The homology of the germinal layers constitutes
in all cases of disputed relationship the court of final appeal. A
new gut, therefore, in any animal can only be formed from hypoblast,
and any theory, such as that advocated in this book, which deals
with the formation of a new gut, and does not form that gut from
pre-existing hypoblast, must of necessity be wrong and needs no
further consideration.

Such is the result of current conceptions — conceptions which to
be valid must be based upon an absolutely clear morphological
definition of the formation of the germinal layers, a definition not
based on their subsequent history and function, but determined solely
by the uniformity of the manner of their origin.

What, then, is a germinal layer ? How can we identify it when it
first arises ? What is the morphological criterion by which hypoblast
can be distinguished from epiblast, or mesoblast from either ?

This is the question put by Braem, in an admirable series of
articles in the Biologisclies Centralblatt, and is one that must be
answered by every worker who bases his views of the process of evolu-
tion upon embryological investigation. As Braem points out, the
germinal layers are definable either from a morphological or physio-
logical standpoint. In the one case they must arise throughout on
the same plan, and whatever be their fate in the adult, they must form
at an early stage structures strictly homologous in all animals. In
the other case the criterion is based on function, and the hypoblast,
for instance, is that layer which is found afterwards to form the defi-
nitive alimentary canal. There is no longer any morphological homo-
logy ; such layers are analogous ; they may be, but are not necessarily,
homologous. Braem gives a sketch of the history of the views held on

THE PRINCIPLES OF EMBRYOLOGY 46 1

the germinal layers, and shows how they were originally a purely
physiological conception, and how gradually such conception changed
into a morphological one, with the result that what had up to that
time been looked upon as analogous structures became strictly homo-
logous and of fundamental importance in decidiug the position of any
animal in the whole animal series.

This change of opinion was especially due to the lively imagina-
tion of Haeckel, who taught that the germinal layers of all Metazoa
must be strictly homologous, because they were all derived from a
common ancestral stock, represented by a hypothetical animal to
which he gave the name Gastraea ; an animal which was formed by
the simple invagination of a part of the blastula, thus giving rise
to the original hypoblast and epiblast, and he taught that throughout
the animal kingdom the germinal layers were formed by such an
invagination of a part of the blastula to form a simple gastrula. If
further investigation had borne out Haeckel's idea, if therefore the
hypoblast was in all cases formed as the invagination of a part of
a single-layered blastula, then indeed the dogma of the homology of
the germinal layers would be on so firm a foundation that no specula-
tion which ran counter to it could be expected to receive acceptance ;
but that is just what has not taken place. The formation of the
gastrula by simple invagination of the single-layered blastula is the
exception, not the rule, and, as pointed out by Brasm, is signifi-
cantly absent in the earliest Metazoa ; in those very places where, on
the Gastraea theory, it ought to be most conspicuous.

Braem discusses the question most ably, and shows again and
again that in every case the true criterion upon which it is decided
whether certain cells are hypoblastic or not is not morphological but
physiological. The decision does not rest upon the answer to the
question, Are these cells in reality the invaginated cells of a single-
celled blastula ? but to the question, Do these cells ultimately form
the definitive alin*sntary canal ? The decision is always based on
the function of the cells, not on their morphological position. Not
only in Braem's paper, but elsewhere, we see that in recent years the
physiological criterion is becoming more and more accepted by
morphologists. Thus Graham Kerr, in his paper on the development
of Lepidosiren, says : " It seems to me quite impossible to define a
layer as hypoblastic except by asking one or other of the two ques-
tions : (1) Does it form the lining of an archenteric cavity ? and (2)

462 THE ORIGIN OF VERTEBRATES

Does it become a certain part of the definitive epithelial lining of
the gut ? "

The appearance of Braem's paper was followed by a criticism from
the pen of Samassa, who agrees largely with Braem, but thinks that
he presses the physiological argument too far. He considers that
morphological laws must exist for the individual development as well
as for the phylogenetic, and finishes his article with the following
sentence, a sentence in which it appears to me he expresses what is
fast becoming the prevailing view : " Mit dem Satz, den man mitunter
lesen kann : ' es muss doch auch fur die Ontogenie allgemeine Ge-
setze geben ' kann leicht Missbrauch getrieben werden ; diese allge-
meinen Gesetze giebt es wohl, aber sie liegen nicht auf flacher Hand
unci bis zu ihrer Erkenntnis hat es noch gute Wege ; das eine kann
man aber wohl heute schon sagen, die Keimblatterlehre gehort zu
diesen allgemeinen Gesetzen nicht."

I conclude, then, that we ought to go back to a time previous to
that of Haeckel and ask ourselves seriously the question, When we
lay stress on the germinal layers and speak of this or that organ arising
from this or that germinal layer, are we thereby adding anything to
the knowledge that we already possess from the study of the anatomy
and physiology of the adult body ? If by hypoblast we only mean
the internal surface or alimentary canal and its glands, etc., and by
epiblast we mean the external surface or skin and its glands, etc.,
while mesoblast indicates the middle structures between the other
two, then I fail to see what advantages we obtain by using Greek
terms to express in the embryo what we express in English in the
adult.

The evidence given by Braem, and it could be strengthened con-
siderably, is conclusive against the morphological importance of the
theory of the germinal layers, and transfers the fundamental impor-
tance of the early embryonic formation, from that of a three-layered
embryo to that of a single-layered embryo — the blastula — from which,
in various ways, the adult animal has arisen.

The derivation of both arthropod and vertebrate from such a
single-layered animal is perfectly conceivable, even though the gut of
the latter is not homologous with the gut of the former. We have
seen that the teachings of embryology, as far as its later stages are
concerned, afford one of the main supports upon which this theory
rests. What, therefore, is required to complete the story is the way

THE PRINCIPLES OF EMBRYOLOGY 46

in which these later stages arise from the blastula stage ; here, as in
all cases, the ontogenetic laws must be in harmony with the phylo-
genetic ; of the latter the most important is the steady develop-
ment of the central nervous system for the upward progress of the
animal race. The study of comparative anatomy indicates the central
nervous system, not the gut, as the keystone of the edifice. So, also, it
must be with ontogeny ; here also the central factor in the formation
of the adult from the blastula ought to be the formation of the
central nervous system, not that of the gut.

Such, it appears to me, is the case, as may be seen from the
following considerations.

The study of the development of any animal can be treated in
two ways : either we can trace back from the adult to the very
beginning in the ovum, or we can trace forward from the fertilized
egg to the adult. Both methods ought to lead to the same result ;
the difference is, that in the first case we are passing from the more
known to the less known, and are expressing the unknown in terms
of the known. In the second case we are passing from the less
known to the more known, and are expressing the known in specula-
tive terms, invented to explain the unknown. What has just been
said with respect to the germinal layers means that, however much
we may study the embryo and try to express the adult in terms of it,
we finally come back to the first way of looking at the question, and,
starting with the adult, trace the continuity of function back to the
first formation of cells having a separate function.

Let us, then, apply this throughout, and see what are the logical
results of tracing back the various organs and tissues from the adult
to the embryo.

The adult body is built up of different kinds of tissues, which fall
naturally, from the standpoint of physiology, into groups. Such
groups are, in the first place —

1. All those tissues which are connected with the central nervous

system, including in that group the nervous system itself.

2. All those tissues which have no connection with the nervous

system.

In the second group the physiologist places all germinal cells, all

blood- and lymph-corpuscles, all plasma-cells and connective tissue

and its derivatives — in fact, all free-living cells, whether in a free

state or in a quiescent, so to speak encysted, condition, such as is

464 THE ORIGIN OF VERTEBRATES

found in connective tissue. In the first group the physiologist
recognizes that the central nervous system is connected with all
muscular tissues, whether striped or unstriped, somatic or splanch-
nic, and that such connection is of an intimate character. Further,
all epithelial cells, either of the outer or inner surfaces, whether
forming special sense-organs and glands, such as the digestive and
sweat-glands, or not, are connected with the nervous system.
Besides these structures, there is another set of organs as to which
we cannot speak definitely at present, which must be considered
separately, viz. all the cells, together with their derived organs, which
line the body-spaces. Whatever may be the ultimate decision as to
this group of cells, it must fall into one or other of the two main
groups.

The members of these two groups are so interwoven with one
another that either, if taken alone, would still give the form of the
body, so that, in a certain sense, we can speak of the body as formed
of two syncytia, separate from each other, but interlaced, of which the
one forms a continuous whole by means of cells connected together
by a fluid medium or by solid threads formed in such fluid medium,
while the other does not form a syncytium in the sense that any cell
of one kind may be connected with any cell of another kind, but a
syncytium of which all the different elements are connected together
only through the medium of the nervous system.

If we choose to speak of the body as made up of two syncytia
in this way, we must at the same time recognize the fundamental
difference in character between them. In the one case the elements
are connected together only by what may be called non-living
material ; there is no direct metabolic activity caused by the action
of one cell over a more distant cell in consequence of such connec-
tion, it is not a true syncytium ; in the second case there is a living
connection, the metabolism of one part is directly influenced by the
activity of another, and the whole utility of the system depends upon
such functional connection.

The tissues composing this second syncytium may be spoken of
as the master-tissues of the body, and we may express this conception
of the building up of the body of the higher Metazoa by saying that
it is composed of a syncytial host formed of the master-tissues, which
contains within its meshes a system of free-living cells, none of
which have any connection with the nervous system. This syncytial

THE PRINCIPLES OF EMBRYOLOGY 465

host is in the adult composed of a number of double elements, a
nerve-cell element, and an epithelial element, such as muscle-cell,
gland-cell, etc., connected together by nerves ; and if such connection
is always present as we pass from the adult to the embryo, if there is
no period when, for example, the neural element exists alone free from
the muscle-cell, no period when the two can be seen to come together
and join, then it follows that when the single-layered blastula
stage is reached, muscle-cell and nerve-cell must have fused together
to form a neuro-muscular cell. Similarly with all the other neuro-
epithelial organs ; however far apart their two components may be
in the adult, they must come together and fuse in the embryo to
form a neuro-epithelial element.

The close connection between muscle and nerve which has always
been recognized by physiologists, together with the origin of muscle
from a myo-epithelial cell in Hydra and other Ccelenterata, led the
older physiologists to accept thoroughly Hensen's views of the neuro-
epithelial origin of all tissues connected with the central nervous
system. Of late years this conception has been largely given up
owing to the statement of His that the nervous system arises from a
number of neuroblasts, which are entirely separate cells, and have at
first no connection with muscle-cells or any peripheral epithelial
cells, but subsequently, by the outgrowing of an axial fibre, find
their way to the muscle, etc., and connect with it. I do not think
that His' statement by itself would have induced any physiologist to
give up the conception of the intimate connection of muscle and
nerve, if the work of Golgi, Ramon y Cajal, and others had not
brought into prominence the neurone theory, i.e. that each element
of the central nervous system is an independent element, without
real connection with any other element and capable of influencing
other cells by contact only. These two statements, emanating as they
did from embryological and anatomical studies respectively, have
done much to put into the background Hensen's conceptions of the
syncytial nature of the motor, neural, and sensory elements, which
make up the master-tissues of the body, and have led to the view
that all the elements of the body are alike, in so far as they are
formed of separate cells each leading an independent existence,
without any real intimate connection with each other.

The further progress of investigation is, it seems to me, bringing
us back to the older conception, for not only has the neuroblast theory

2 H

466 THE ORIGIN OF VERTEBRATES

proved very difficult for physiologists to accept, but also Graham
Kerr, in his latest papers on the development of Lepidosiren, has
shown that there is continuity between the nerve-cell and the muscle-
cell from the very first separation of the two sets of elements ; in
fact, Hensen is right and His wrong in their respective interpretation
of the earliest stages of the connection between muscle and nerve.
So also, it seems to me, the intimate connection between the meta-
bolism of the gland-cell, as seen in the submaxillary gland, and the
integrity of its nervous connection implies that the connection
between nerve-cell and gland-cell is of the same order as that between
nerve-cell and muscle-cell. Graham Kerr also states in his paper
that from the very commencement there is, he believes, continuity
between nerve-cell and epithelial cell, but so far he has not obtained
sufficiently clear evidence to enable him to speak positively on this
point.

Further, according to the researches of Anderson, the cells of the
superior cervical ganglion in a new-born animal will continue to
grow healthily as long as they remain connected with the periphery,
even though entirely separated from the central nervous system by
section of the cervical sympathetic nerve, and conversely, when
separated from the periphery, will atrophy, even though still con-
nected with the central nervous system. So, also, on the sensory
side, Anderson has shown that the ganglion-cells of the posterior
root-ganglion will grow and remain healthy after separation of the
posterior roots in a new-born animal, but will atrophy if the peripheral
nerve is cut, even though they are still in connection with the central
nervous system. Further, although section of a posterior root in the
new-born animal does not affect the development of the nerve-cells
in the spinal ganglion, and of the nerve-fibres connecting the
posterior root-ganglion with the periphery, it does hinder the
development of that part of the posterior root connected with
the spinal ganglion.

These experiments of Anderson are of enormous importance, and
force us, it seems to me, to the same conclusion as that to which he
has already arrived. His words are (p. 511): "I suggest, therefore,
that the section of peripheral nerves checked the development of
motor and sensory neurones, not because it blocked the passage of
efferent impulses in the first case and the reception of stimuli from
the periphery in the second, but for the same reason in both cases,

THE PRINCIPLES OF EMBRYOLOGY 467

viz. that the lesion disturbed the chemico-physical equilibrium of an
anatomically continuous (neuro-muscular or neuro-epithelial) chain
of cells, by separating the non-nervous from the nervous, and that
the changes occurring in denervated muscle, which I shall describe
later (and possibly those in denervated skin), are in part due to the
reciprocal chemico-physical disturbance effected in these tissues by
their separation from the nervous tissues ; also that the section of
the posterior roots checked the development of those portions of
them still attached to the spinal ganglia, because the chemico-
physical equilibrium in those processes is maintained not only by
the spinal ganglion-cells, but also by the intra-spinal cells with which
these processes are anatomically continuous."

What is seen so strikingly in the new-born animal can be seen
also in the adult, and in Anderson's paper references are given
to the papers of Lugaro and others which lead to the same
conclusion.

These experiments seem to me distinctly to prove that the
connection between the elements of the peripheral organ and the
proximate neurone is more than one of contact.

We can, however, go further than this, for, apart from the
observations of Apathy, there is direct physiological evidence that
the vitality of other neurones besides the terminal neurunes is
dependent upon their connection with the peripheral organ, even
though their only connection with the periphery is by way of the
terminal neurone. Thus, as is seen from Anderson's experiments,
section of the cervical sympathetic nerve in a very young animal
causes atrophy of many of the cells in the corresponding intermedio-
lateral tract, cells which I supposed gave origin to all the vaso-
constrictor, pilomotor, and sweat-gland nerves. A still more striking
experiment given by Anderson is the effect of the removal of the
periphery upon the inedullation of those efferent fibres which arise
from these same spinal cells, for, as he has shown, section of the
nerves from the superior cervical ganglion to the periphery in a very
young animal delays the medullation in the fibres of the cervical
sympathetic — that is, in preganglionic fibres which are not directly
connected with the periphery but with the terminal neurones in the
superior cervical ganglion. So also on the afferent side a sufficiently
extensive removal of sensory field will cause atrophy of the cells of
Clarke's column, so that, just as in the case of the primary neurones,

468 THE ORIGIN OF VERTEBRATES

the secondary neurones show by their degenerative changes the
importance of their connection with the peripheral organs.

In this way I can conceive the formation of a series of both
efferent and afferent relays in the nervous system by proliferation of
the original neural moiety of the neuro-epithelial elements, every
one of which is dependent upon its connection with the peripheral
epithelial elements for its due vitality, the whole system being a
scheme for co-ordination of a larger and larger number of peripheral
elements. Thus the cells of the vasomotor centre are in connection
with the whole system of segmental vaso-constrictor centres in the
lateral horns of the thoracic region of the cord, so that to cause
atrophy of these cells a very extensive removal of the vascular
system would be required. Each of the segmental centres in the
cord supplies a number of sympathetic segments, the connection
with all of which would have to be cut in order to ensure complete
removal of the connection of each of its cells with the periphery, and
finally each of the cells in the sympathetic ganglia supplies a number
of peripheral elements, all of which must be removed to ensure com-
plete severance.

Thus, if we take any arbitrary number, such as 4, to represent
the number of peripheral organ-elements with which each terminal
neurone is connected, and suppose that each neurone has proliferated
into sets of 4, then a cell of the third order, such as a cell of the
vasomotor centre, would recpuire the removal of 64 peripheral elements
to cause its complete separation from the periphery, one of the
second order (a cell of the thoracic lateral horn) 16 elements, one of
the first order (a cell of a sympathetic ganglion) 4 elements.

Such intimate inter-relationship between the neurones, both
afferent and efferent, and their corresponding peripheral organs does
not imply that all nerve-cells are necessarily as closely dependent
upon some connection with the periphery, for just as the proliferation
of epithelial or muscle-cells forms an epithelial or muscular sheet,
the elements of which are so loosely, if at all, connected together that'
their metabolism is in no way dependent upon such connection, so
also a similar proliferation of the neural elements may form con-
nections between nerve-cell and nerve-cell of a similarly loose
nature.

It is this kind of proliferation which, in my opinion, would bind
together the separate relays of efferent and afferent neurones, and

THE PRINCIPLES OF EMBRYOLOGY 469

so give origin to reflex actions at different levels. Such neurones
would not be in the direct chain of either the afferent or efferent
neurones, and so not directly connected with the periphery, and
could therefore be removed without affecting the vitality of either
the efferent or afferent chain of neurones. In other words, the
vitality of the cells on the efferent side ought not to be dependent
on the integrity of the reflex arc. With regard to the development
of the anterior roots, Anderson has shown that this is the case, for
section of all the posterior roots conveying afferent impulses from
the lower limb in a new-born animal does not hinder the normal
development of the anterior roots supplying that limb. Also Mott,
who originally thought that section of all the posterior roots to a
limb caused atrophy of the corresponding anterior roots, has now
come to the same conclusion as other observers, and can find no
degeneration on the efferent side due to removal of afferent impulses.

Again, the process of regeneration after section of a nerve is
not in favour of the neuroblast theory. There is no evidence that
the cut end of a nerve can grow down and attach itself to a
muscular or epithelial element without the assistance of a nerve
tube down which to grow. When the cut nerves connected with
the periphery degenerate, that applies only to the axis-cylinder
and the medullary sheath, not to the neurilemma ; the connective
tissue elements remain alive and form a tube into which the growing
axon finds its way, and so is conducted to the end-plate or end-
organ of the peripheral structure.

Possibly, as suggested by Mott and Halliburton, the products
of degeneration of the axis-cylinder and medullary sheath stimulate
these connective tissue sheath-cells into active proliferation, and
so bring about the great multiplication of cells arranged as cell-
chains, which are so often erroneously spoken of as forming the
young nerves. These sheath-cells are then supposed to re-form
and secrete a pabulum which is important for the process of re-
generation of the down-growing axis-cylinder and medullary sheath.
Without such pabulum regeneration does not take place, as is
seen in the central nervous system, where the sheath of Schwann
is absent.

A^ain, it is becoming more and more doubtful whether the
peripheral terminations of nerves are ever really free. As far as
efferent nerves are concerned the nervous element may entirely

470 THE ORIGIN OF VERTEBRATES

predominate over the muscular or glandular, as in the formation of the
electric organs of the Torpedo and Malapterurus, but still the final
effect is produced by the alteration of the muscle or gland-cell. On
the afferent side especially free nerve-terminations are largely recog-
nized, or, as in Barker's book, nerves are spoken of as arising in
connective tissue. Thus the numerous kinds of special sense-organs,
such as Pacinian bodies, tendon-organs, genital corpuscles, etc., are
all referred to by Barker under the heading of " sensory nerve
beginnings in mesoblastic tissues." Yet the type of these organs
has been known for a long time in the shape of Grandry's corpuscles
or the tactile corpuscles in the duck's bill, where it has been proved
that the nerve terminates in special large tactile cells derived from
the surface-epithelium.

So also with all the others, further investigation tends to put
them all in the same category, all special sensory organs originating
from a localized patch of surface- epithelium. Thus Anderson has
shown me in his specimens how the young Pacinian body is
composed of ro-ws of epithelial cells, into each of which a twig
from the nerve passes. He has also shown me how, in the case of
the tendon-organ, each nerve-fibre passes towards the attachment of
the tendon and then bends back to supply the tendon-organ, thus
iudicating, as he suggests, how the nest of epithelial cells has
wandered inwards from the surface to form the tendon-organ. Again,
Meissner's corpuscles and Herbst's corpuscles are evidently referable
to the same class as those of Grandry and Pacini.

Yet another instance of the same kind is to be found in the
chromatophores of the frog and other animals which are under the
influence of the central nervous system and yet have been supposed
by various observers to be pigmented connective tissue cells. The
most recent wrork of Leo Loeb and others has conclusively shown
that such cells are invariably derived from the surface-epithelium.

Finally, in fishes we find the special sense-organs of the lateral
line and other accessory sensory organs, all of which are indisputably
formed from modified surface epithelial cells.

The whole of this evidence seems to me directly against Barker's
classification of sensory nerve-beginnings in mesoblastic tissues ; in
none of these cases are we really dealing with free nervous tissue
alone, the starting point is always a neuro-epithelial couple.

We may then, I would suggest, look upon the adult as formed of

THE PRINCIPLES OE EMBRYOLOGY tf \

a neural syncytium, which we may call the host, which carries with
it in its meshes a number of free cells not connected with the nervous
system. If, then, we confine our attention to the host and trace back
this neural syncytium to its beginnings in the embryo, we see that,
from the very nature of the neuro-epithelial couple, each epithelial
moiety must approach nearer and nearer to its neural moiety, until
at last it merges with it ; the original neuro-epithelial cell results,
and we must obtain, as far as the host is concerned, a single-layered
blast ula as the origin of all Metazoa. It follows, further, that there
must always be continuity of growth in the formation of the host,
i.e. in the formation of the neuro-epithelial syncytium ; that there-
fore cells which have been previously free cannot settle down and
take part in its formation, as, for instance, in the case of the formation
of any part of the gut- epithelium or of muscle-cells from free-living
cells.

Further, since the neural moiety is the one element common to
all the different factors which constitute the host, it follows that the
convergence of each epithelial moiety to the neural moiety, as we
pass from the adult to the embryo, is a convergence of all outlying
parts to the neural moiety, i.e. to the central nervous system, if there
is a concentrated nervous system. Conversely, in the commencing
embryo the place from which the spreading out of cells takes place,
■i.e. from which growth proceeds, must be the position of the central
nervous system, if the nervous system is concentrated. If the nervous
system is diffuse, and forms a general sub-epithelial layer, then the
growth of the embryo would take place over the whole surface of
the blastula.

Turning now to the consideration of the second group of tissues,
those that are not connected with the central nervous system, we
find that they include among them such special cells as the germinal
cells, free cells of markedly phagocytic nature, and cells which were
originally free and phagocytic, but have settled down to form a
supporting framework of connective tissue, and are known as plasma-
cells. In the embryo we find also in many cases free cells in the
yolk, forming more or less of a layer, which function as phagocytes
and prepare the pabulum for the fixed cells of the growing embryo ;
these cells are known by the name of vitellophags, and in meroblastic
vertebrate eggs form somewhat of a layer known by the name of
periblast. Such cells must be included in the second group, and,

472 THE ORIGIN OF VERTEBRATES

indeed, have been said again and again to give origin to the free-
living blood- corpuscles of the adult. In other cases they are said to
disintegrate after their work is done.

In the adult the free-living lymphocytes and hremocytes reproduce
themselves from already existing free-living cells, but as we pass back
to the embryo there comes a time, comparatively late in the history
of the embryo, when such free-living cells are not found in the fluids
of the body, and they are said to arise from the proliferation and
setting free of cells which form a lining epithelium. Such formation
of leucocytes has been especially described in connection with the
lining epithelium of the ccelomic cavities, as stated in Chapter XII.,
so that anatomists look upon the origin of these free cells as being
largely from the ccelomic epithelium, or mesothelium, as Minot
calls it.

Then, again, the free cells which form the germinal cells can be
traced back to a germinal epithelium, which also is part of the cadorn.
Thus the suggestion arises that in the embryo a cellular lining is
formed to a ccelomic cavity (mesothelium) composed of cells which
have no communication with the nervous system, and are capable of a
separate existence as free individuals, either in the form of germinal
cells or of lymphocytes, hamiocytes, and plasma-cells, so that these
latter free cells may be considered as living an independent existence
in the body, and ministering to it in the same sense as the germ-cells
live an independent existence in the body. Again, the function of this
mesothelium apart from the germ -cell is essentially excretory and
phagocytic. It is the cells of the excretory organs as well as the
lymphocytes which pick up carmine-grains when injected. It is the
cells of the modified excretory organs, as mentioned in Chapter XII.,
which, according to Kowalewsky and others, give origin to the free
leucocytes.

We see, then, that the conception of a syncytial neuro-epithelial
host holding in its meshes a number of free cells leads directly to the
questions : What is the ccelom ? To which category does its lining
membrane belong? and further, also, What is the origin of these
free cells ?

The Metazoa have been divided into two great groups — those
which possess a ccelom (the Ccelomata ; Lankester's Gelonioccela)
and those which do not (Ccelenterata ; Lankester's Enteroccela). As
an example of the latter we may take Hydra, because it is a very

THE PRINCIPLES OF EMBRYOLOGY 473

primitive form, and because its development has been carefully
worked out recently by Brauer.

In Hydra we find a dermal layer of cells and an inner layer of
cells separated by a gelatinous mass known as mesogloea ; in this
mass between the dermal and inner layers scattered cells are found,
the interstitial cells. Now, according to Brauer the position of the
germ in Hydra is the interstitial cell-layer. One cell of the ovarium
becomes the egg-cell, the others have their substance changed into
yolk-grains, forming the so-called pseudo-cells, and as such afford
pabulum to the growing egg-cell. Thus we see that in between the
dermal and gastral layer of cells a third layer of cells is found, com-
posed of free living germ- cells, some of which, by the formation of
yolk-granules, become degraded into pabulum for their more favoured
kinsfolk. These interstitial cells are said to arise from the dermal
layer, or ectoderm, but clearly, as in other cases, germ-cells constitute
a class by themselves and cannot be spoken of as originating from
ectoderm-cells or from hypoderm-cells.

So also in Porifera, Minchin states : " In addition to the collared
cells of the gastral layer, and the various cell-elements of the dermal
layer, the body-wall contains numerous wandering cells or amcebo-
cytes, which occur everywhere among the cells and tissues. Though
lodged principally in the dermal layer, they are not to be regarded
as belonging to it, but as constituting a distinct class of cells by
themselves. They are concerned probably with the functions of
nutrition and excretion, and from them arise the genital products."
Further (p. 31) : " At certain seasons some of these cells become
germ-cells; hence the wandering cells and the reproductive cells
may be included together under the general term archieocytes." Also
(p. 51): " The mesoglcea is the first portion to appear as a structure-
less layer between the dermal and gastral epithelia, and is probably
a secretion of the former."

He also points out that in these, the very lowest of the Metazoa,
the separate origin of these archoeocytes can be traced back to a very
early period of embryonic life. Thus in Clathrina blanca the ovum
undergoes a regular and total cleavage, resulting in the formation of
a hollow ciliated blastula of oval form. At one point, the future
posterior pole of the larva, are a pair of very large granular cells with
vesicular nuclei, which represent undifferentiated blastomeres and
are destined to give rise to the arclucocytes, and, therefore, also to the

474 THE ORIGIN OF VERTEBRATES

sexual cells of the adult. Thus, as he says, from the very earliest
period a distinction is made between the " tissue-forming " cells (my
syncytial host) and the archreocytes.

We see, then, that the origin of all these free-living cells can be
traced back to the very earliest of the Metazoa. Here between the
dermal and gastral layers a gelatinous material, the mesoghea is
secreted by these layers. This material is non-living, non-cellular.
In it live free cells which may either be germ-cells, amcebocytes,
or ' collencytes ' (connective tissue cells). If this mesogloea were a
fluid secretion, then we should have a tissue of the nature of blood
or lymph ; if it were solid, then we should have the foundation of
connective tissue, cartilage, and bone.

From this primitive tissue it is easy to see how the special
elements of the vascular, lymphatic, and skeletal tissues gradually
arose, the matrix being provided by the cells of the syncytial host
and the cellular elements by the archfeocytes. In fact, we have no
right to speak of these lowest members of the Metazoa as not being
triploblastic, as possessing nothing corresponding to mesoblast, for
in these free cells in the mesogloea we have the origin of the
mesenchyme of the higher groups. Thus Lankester, talking of
mesenchyme, says : " I think we are bound to bring into considera-
tion here the existence in many Ccelentera of a tissue resembling
the mesenchyme of Ccelomocoela. In Scyphomedusre, in Ctenophora,
and in Anthozoa, branched fixed and wandering cells are found
in the mesogloea which seem to be the same thing as a good
deal of what is distinguished as mesenchyme in GVelomoccela.
These appear to be derived from both the primitive layers ; some
produce spicules, others fibrous substance, others again seem to be
amcebocytes with various functions. It appears to be probable that,
though it may be necessary to distinguish other elements in it, the
mesenchyme of Ccelomocoela is largely constituted by cells, which
are the mother-cells of the skeletotrophic group of tissues, and are
destined to form connective tissues, blood-vessels, and blood."

Thus we see that the earliest Metazoa were composed of a dermal
and gastral epithelium, with a sub-epithelial nervous system con-
necting the parts together, which formed, as it were, a host, carrying
around free living cells of varying function, all of which may be
looked on as derived from arclueocytes, i.e. germ-cells. From these
the ccelomatous animals arose, and here also we find, according to

THE PRINCIPLES OF EMBRYOLOGY 475

present-day opinion, that the ccelom arose in the first place in the
very closest connection with the germ-cells or gonads. Thus
Lankester, in his review of the history of the ccelom, states : —

"The numerous embryological and anatomical researches of the
past twenty years seem to me to definitely establish the conclusion
that the ccelom is primarily the cavity, from the walls of which the
gonad cells (ova or spermata) develop, or which forms around those
cells. We may suppose the first ccelom to have originated by a
closing or shutting off of that portion of the general archenteron of
Enteroccela (Ccelentera), in which the gonads developed as in Aurelia
or as in Ctenopkora. Or we may suppose that groups of gonad
mother cells, having proliferated from the endoderm, took up a
position between it and the ectoderm, and there acquired a vesicular
arrangement, the cells surrounding the cavity in which liquid
accumulated.

"The ccelom is thus essentially and primarily (as first clearly
formulated by Hatschek) the perigonadial cavity or gonoccel, and
the lining cells of gonadial chambers are ccelomic epithelium. In
some few groups of Ccelomoccela the cceloms have remained small
and limited to the character of gonoccels. This seems to be the case
in the Xemertina, the Planarians, and other Platyhelmia. In some
Planarians they are limited in number, and of individually large size ;
in others they are numerous."

When Lankester says that " the lining cells of gonadial chambers
are ccelomic epithelium," that is equivalent to saying that the lining
cells of the ccelom form an epithelium which was originally gonadial,
provided that, as seems to me most probable, his second suggestion,
of the ccelom being formed from gonadial mother-cells which have
taken up an intermediate position between endoderm and ectoderm
and there acquired a vesicular arrangement, is the true one. It does
not seem to me possible to conceive of the gonads arising from cells
of the epiblast or of the hypoblast, in the sense that such cells are
differentiated cells belonging to a layer with a definite meaning.
When we consider that the gonad gives origin to the whole of a
new individual, that in the protozoan ancestors of the Metazoa their
ultimate aim and object was the formation of gonads, it seems a wrong
conception to speak of the gonads as formed from cells belonging
either to the gut-wall or to the external epithelium. The gonads
must stand in a category by themselves ; they represent a whole,

476 THE ORIGIN OF VERTEBRATES

while the other cells represent only a part ; they cannot therefore be
derived from the latter. They may, and indeed do, give rise to cells
of a subordinate character, but they cannot rightly be spoken of as
derived from such cells. The very fact mentioned by Lankester, that
in the lowest ccelomatous Metazoa, the Platyhelminthes, the cceloms
are limited to the character of simple gonoccels, strongly points to
the conclusion that all the ccelomic cells were originally of the nature
of gonadial cells, and therefore free-living and independent of the
rest of the cells of the body. Whether the germ-cells appear, as in
Hydra, to be derived from the ectoblast, or, as is usually stated, from
the endoblast, in neither case ought they to be classed with the internal
or external epithelium ; they are germ-cells, and the epithelium which
they form is neither epiblastic nor hypoblastic, but germinal, forming
originally a simple gonoccele, afterwards, in the higher forms, the
ccelom with its cells of various function. Thus, to quote again from
Lankester, " The ccelomic fluid and the ccelomic epithelium, as well
as the floating corpuscles derived from that epithelium, acquire special
properties and importance over and above the original functions
subservient to the maturation of the gonadial cells . . . the most
important developments of the ccelom are in connection with the
establishment of an exit for the generative products through the
body-wall to the outer world, and further in the specialization of
parts of its lining epithelium for renal excretory functions."

Such exits led very early to the formation of ccelomoducts, which
are true outgrowths of the ccelom itself (p. 14) : " The ccelomoducts
and the gonoccels of which they are a part, frequently acquire a renal
excretory function, and may retain both the function of genital con-
duits and of renal organs, or may, where several pairs are present
(metamerized or segmented animals), subserve the one function in
some segments of the body, and the other function in other segments."

The origin of the ccelom and its derivatives from a germinal
membrane, as suggested by Lankester, appears to me most probable,
and, if true, it carries with it conclusions of far-reaching importance,
for it necessitates that all the cells which line true ccelomic cavities,
and their derivatives, belong to the category of free-living cells, and
are not connected with the nervous system. The cells in question
are essentially those which line serous cavities and those which form
excretory glands such as the kidneys. In the latter organ we ought
especially to be able to obtain a clear answer to this question, for is

THE PRINCIPLES OF EMBRYOLOGY 477

it not a "land which secretes into a duct and rnight therefore be
expected to be innervated in the same way as other secretory glands ?
Although there is a strong prima facie presumption in favour of
the existence of renal secretory nerves, yet according to the universal
opinion of physiologists no evidence in favour of such nerves has
hitherto been found ; all the phenomena of excretion of urine
consequent on nerve stimulation are explicable by the action of
nerves on the renal vessels, not on the renal cells. Not only is the
physiological evidence negative up to the present time, but also, I
think, the histological. On the one hand, Eetzius has failed to find
nerve-connections with kidney-cells ; on the other, Berkley has
obtained such evidence with the Golgi method, but failed entirely
with methylene blue. I do not myself think that the evidence of
the Golgi method alone is sufficient without corroboration by other
methods, and, in any case, Berkley's evidence does not show the
nerve-fibres terminating in the kidney-cells, in the same way as can
be shown by modern methods to exist in the case of epithelial cells
of the surface, etc. Quite recently another paper on this subject has
appeared by Smirnow, who appears to have obtained better results
than those given by Berkley.

Apart from these physiological and histological considerations,
this question is also dependent upon the nature of the development
of the excretory organs, for, according to Lankester, all excretory
organs may be divided into the two classes of nephridial organs and
crelomostomes, of which the former are largely derived from epiblast.
We should, therefore, expect to find secretory nerves to nephridial
organs, though possibly not to ccelomostomes. The kidneys of the
Mammalia are supposed to be true ccelomostones, although, according
to Goodrich's researches, the excretory organs in Amphioxus are
solenocytes, i.e. true nephridia.

As to the lining epithelium of the peritoneal, pleural, and
pericardial cavities — i.e. the mesothelium — there is no definite
evidence that these cells are provided with nerves. Such surfaces
are remarkably insensitive in the healthy condition, and the pain
in such cavities is essentially a pressure phenomenon and referable
to special sense-organs, such as Pacinian bodies, etc., rather than
to the mesothelium itself.

These sense-organs are identical in structure with those in the
skin, and, as Anderson has shown, the nerves of these organs

47 8 THE ORIGIN OF VERTEBRATES

medullate at the same time as those in the skin, and both obtain
their medullary sheaths earlier than any other nerves, whether
afferent or efferent. However difficult it may be to explain this fact,
only one conclusion seems to me possible — these Pacinian bodies, like
the skin Pacinians, originate from a nest of surface epithelial cells, a
conclusion which is extremely probable on my theory of the origin of
vertebrates, but not, as far as I can see, on any other.

At the present moment the weight of evidence is, to my mind,
in favour of the lining endothelium of the ccelomic cavities being-
composed of free cells, unconnected with the nervous system rather
than the reverse, but I must confess that the question is undecided.
If it be true that the coelomic lining is partly enterocoelic and partly
gonoccelic, as Lankester teaches, then it would be natural that its
cells should be in connection with the nervous system, to some
extent at all events. This view is, however, based on very slender
foundations. If the mesothelium is composed of cells capable of
becoming free, it cannot give rise to the skeletal muscles, and it
cannot therefore be right to speak of the skeletal muscles as
derived from the lining cells of a part of the primary ccelom.
The phylogenetic history of the musculature of the different
animals points strongly to its intimate connection with and deriva-
tion from surface epithelial cells rather than from coelomic mesothelial
cells. Thus in the ccelenterates, as seen in Hydra, the muscular
layer arises directly from a modification of the surface epithelial
cells ; and right up to the annelids, even to the highest form in the
Polychaita, we still see it stated that the musculature, both circular
and longitudinal, arises from the ectoderm. In the Oligochseta and
Hirudinea, according to Bergh, there are five rows of teloblasts on
each side, of which four are ectodermic and give rise to the nerve-
ganglia and the circular muscles, while one is mesoblastic and forms
the nephridial organs and the longitudinal muscles. (The latter
statement is, according to Bergh, well known, and is not particularly
shown by him. These longitudinal muscle-bands always lie close
against the nervous system at their first formation, and may well
have been derived in connection with it.)

It is apparently only in the Vertebrata that the lining cells of the
cojlomic cavity are definitely stated to give origin to the body-muscu-
lature, and taking into account on the one hand the evidence of
Graham Kerr as to the intimate connection between nerve-cell and

THE PRINCIPLES OF EMBRYOLOGY 479

muscle- cell from the very beginning, and on the other the manner in
■which all the skeletal muscles of the adult are lined with a lymphatic
endothelium, I am strongly inclined to believe that at the closing
up of the lnyoccele, when the myomere separates from the mesomere,
the lining cells remain scattered in among the forming muscle-cells
and form the ultimate lymphatic tissue of the muscles. If this
is really so, then the evidence in favour of the mesothelium being
composed of free cells not connected with the nervous system
would be much strengthened for, on the one hand, an intimate
relation exists between the connective tissue cells and the endo-
thelium of the roots of the lymphatic vessels, a relation which,
according to Virchow, has rendered it impossible to draw any sharp
line of distinction between the two ; and, on the other, the lymphatic
endothelium merges into the lining cells of the great serous cavities
of the body.

It is impossible to conceive of an animal possessing a nervous
system which is not in connection with sensory and muscular
tissues ; an isolated nerve-cell is a meaningless possession ; but it is
equally natural to conceive of a germ-cell being isolated, capable of
living an independent existence. Such a difference between the two
kinds of tissues must have existed from the very commencement of
the Metazoa, so that we must, it seems to me, imagine that in the
formation of the Metazoa from the Protozoa the whole of the body
of the latter did not break up into a mass of separate gonads, each
capable of becoming a free-living protozoan similar to its parent, but
that a portion proliferated into a multinucleated syncytium while
the remainder formed the free-living gonads. This multinucleated
syncytium, or host, as it might be called, would still continue to
exist for the purpose of carrying further afield the immortal gonads,
which need no lcnger be all shed at one time.

In such an animal as Vol vox gldbator we have an indication of
the very kind of animal postulated as connecting the single-celled
Protozoa and the multi-cellular Metazoa, for it consists of a many-
celled case which forms a hollow sphere, each of the cells being
provided with flagella for the purpose of locomotion of the sphere,
except a certain number which are not flagellated ; the latter leave
the case to swim freely in the fluid contained within the sphere, and
forming spermaries and ovaries, conjugate, maturate, and then are set
free by the rupture of the encircling locomotor host.

480 THE ORIGIN OF VERTEBRATES

This conception of the predecessors of the Metazoa being com-
posed of a mortal host, holding within itself the immortal sexual
products, leads naturally to the idea of the separate development
of the host from that of the germ-cells ab initio, so that the
study of the development of the Metazoa means the study of two
separate constituents of the metazoan individual — on the one
hand, the elaboration of the elements forming the syncytial host,
on the other, of those derived from the free-living independent germ-
cells. The elaboration of the host means the differentiation of the
protoplasm into epithelial, muscular, and nervous elements, by means
of which the gonads were carried further afield and their nourishment
as well as that of the host ensured.

The role of the nervous system as the middleman between internal
and external muscular and epithelial surfaces was, I imagine, initiated
from the very earliest time. The further evolution of the host con-
sisted in a greater and greater differentiation and elaboration of this
neuro-epithelial syncytium, with the result of a steadily increasing
concentration and departmental centralization of the main factor of
the syncytium ; in other words, it led to the origin and elaboration
of a central nervous system. In the interstices of this syncytium
the gonads were placed, and at first, doubtless, the life of the host
ended when all the germ-cells had been set free. ' Eeproduce
and die ' was, I imagine, the law of the Metazoa at its earliest
origin, and throughout the ages, during all the changes of evo-
lution, the reminiscence of such law still manifests itself even up
to the highest forms as yet reached. With the differentiation of
the syncytial host there came also differentiation of the free-living
gonads, so that only some of them attained to the perfection of
independent existence, capable of continuing the species ; while others
became subordinate to the first and provided them with pabu-
lum, manufacturing within themselves yolk-spherules, and thus in
the shape of yolk-cells ministered to the developing egg-cell. Thus
arose a germinal epithelium of which only a few of the elements
passed out of the host as perfect individuals, the remainder being-
utilized for the nutrition of these few. Such yolk-cells of the
germinal epithelium would still, however, retain their character as
free cells totally independent of the syncytial host, and, situated as
they were between the internal and external epithelium, capable of
amoeboid movement, would naturally have their phagocytic action

THE PRINCIPLES OF EMBRYOLOGY 48 1

utilized either as yolk-cells for the providing of pabulum to the egg-
cell, or as excretory cells for the removal and rendering harmless of
deleterious products of all kinds. Thus the free cells of the body
would become differentiated into the three classes of germ-cells,
yolk-cells, and excretory cells.

Further, the mass of gonads, which originally occupied so large
a space within the interior of the host, necessarily, as the tissues of
the host differentiated more and more, took up less and less space in
proportion to the whole bulk of the host and formed a germinal mass
of cells between the outer and inner epithelial layers. This germinal
mass formed an epithelium, some of the members of which acted as
scavengers for the inner and outer layers of the host, with the result
that fluid accumulated between the two parts of the germinal
epithelium in connection respectively with the external and internal
epithelial surfaces of the host, and thus led to the formation of a
gonocoele, which, by obtaining an external opening, a ccelomostome,
gave origin to the ccelom.

Again, with the longer life of the host, the setting free of the
gonads no longer necessitating the destruction of the host, and also
the gonads themselves recpiiiring a longer and longer time to be fed
up to maturity, the bulk and complexity of the whole organism
increased and special supporting structures became a necessity. The
host itself could and did provide these to a certain extent by secre-
tions from its epithelial elements, but the intermediate supports were
provided by the system of phagocytic cells utilizing the fluids of the
body, at first in the shape of plasma-cells able to move from place to
plaee, then settling down to form a connective tissue framework, and,
later on, cartilage and bone.

So also were gradually evolved the whole of the endothelial
structures ; the lymph-cells, blood-cells, etc., all having their origin
from the free cells of the body, which themselves originated in
the extension of a germinal epithelium. Just as in a bee-hive the
egg-cells may form the fully developed sexual animal, whether drone
or queen bee, or the asexual host of workers, so in the body of the
Metazoa the free cells may form either male or female germ-cells
spermatozoa, or ova, or a host of workers, scavengers, repairers, food-
providers, all useful to the community, all showing their common
origin by their absolute independence of the nervous system.

Two points of great importance follow from this method of looking

2 I

482 THE ORIGIN OF VERTEBRATES

at the problem. First, the evolution of the animal kingdom means
essentially the evolution of the host, for that is what forms the
individual ; secondly, as the host is composed of a syncytium, the
common factor of whose elements is the neural moiety, it follows
that the tissue of central importance for the evolution of the host
must be, as indeed it is, the nervous system. Further, seeing that
the growth of the individual means the orderly spreading out of the
epithelial moiety away from the neural moiety, it follows that the
germ-band or germ-area from which growth starts must be in
the position of the nervous system. If then, the nervous system in
the animal is a concentrated one, then the growth will emanate
from the position of such nervous system. If, on the other hand,
the nervous system is diffused, then the growth will also be diffused.

In this book I have throughout argued that the ancestors of verte-
brates belonged to a great group of animals which gave origin also to
Limulus and scorpion-like animals ; it is therefore instructive to see
what is the nature of the development of such animals. For this
purpose I will take the development of the scorpion, as given by
Brauer, for he has worked out its development with great thorough-
ness and care. His papers show that the segmentation is discoidal,
and results in an oval blastodermic area lying on a large mass of yolk.
Very early there separates out in this area genital cells and yolk-
cells, which latter move freely into the yolk and prepare it into a
fluid pabulum for the nutrition of the cells of the embryonic shield
or germ-band. These free yolk-cells do not take part in the formation
of the germinal layers, nor does the endoderm when formed give
origin to free yolk-cells.

The cells of the germ-band form a small compact area, in which
by continual mitosis the cells become more than one-layered, and soon
it is found that those cells which lie close against the fluid pabulum
form a continuous layer and absorb the nutritious material for them-
selves and the rest of the embryo. While this area is thus increasing
in thickness by continuous development, the group of genital cells
remains always apart, increasing in number, but being always in a
state of isolation from the cells of the rest of the growing area. Thus
from the very first Brauer's observations on the development of
the scorpion point to the formation of a syncytial host containing
separate genital cells. The continuous layer of cells against the
fluid pabulum, which is already functioning as a gut, and may

THE PRINCIPLES OF EMBRYOLOGY 483

therefore be called hypoblast, spreads continuously over the yolk, as
also does the surface epithelial layer, or epiblast. Such spreading
is always a continuous one for both surfaces, so that the yolk is
gradually enclosed by a continuous orderly growth from the germ-
band, and not by the settling down of free cells in the yolk here
and there to form the gut-lining. This steady orderly development
proceeds owing to the nourishment afforded by the activity of the
free cells or vitellophags and the absorbing power of the hypoblast,
a steady growth round the yolk which results in the formation of the
gut-tube, the outer covering and all the muscular and excretory
organs. Where, then, is this starting-point, this germ-band from
which the whole embryo grows ? It forms the mid ventral area of
the adult animal, it corresponds exactly to the position of the
central nervous system. The whole phenomenon of embryonic
growth in the scorpion is exactly what must take place on the
argument deduced from the study of the adult that the animal
arises as a neuro-epithelial syncytium, and we see that that layer of
cells which is situated next to the food-material forms the alimen-
tary tube. It is not a question whether such layer is ventral or
dorsal to the neural cells, but whether it is contiguous to or removed
from the food-material.

Take, again, a meroblastic vertebrate egg as of the bird. Again we
find free cells passing into the yolk to act as vitellophags, the so-called
periblast cells ; again we see that the embryo starts from a germ-
band or embryonic shield, and spreads from there continuously and
steadily ; again we see that the layer of cells which lies against the
yolk absorbs the fluid pabulum for the growing cells ; again we see
that the area from which the whole process of growth starts is that
of the central nervous system, and again we see that those cells
which are contiguous to the food form the commencing gut, and are
therefore called hypoblast, though in this case they are ventral not
dorsal to the neural layer.

The comparison of these two processes shows that there is one
common factor, one thing comparable in the two, one thing that is
homologous and is the essential in the formation of that part of the
animal which I have called the host, and that is the central nervous
system. Whether the epithelial layer which lies ventrally to it or the
one that is dorsal forms the gut depends upon the position of the
food-mass. Where the food is, there will be the absorbing layer.

484 THE ORIGIN OF VERTEBRATES

Where the food is not, there will be no gut formation, whatever may
have been the previous history of that layer. If, then, we suppose,
as I do, that the vertebrate arose from a scorpion-like animal without
any reversal of dorsal and ventral surfaces, and that the central
nervous system remained the same in the two animals, then the
comparison of the development of the two embryos shows that the
one would be derived from the other if the yolk-mass shifted from
the dorsal to the ventral side of the nervous system. This would
leave the dorsal epithelial layer of the original syncytium free from
pabulum ; it would no longer form the definite gut, but it would
still tend to form itself in the same manner as before, would still grow
from a vcntrally situated germ-band dorsalwards to form a tube, ivoidd
recapitulate its fast history, and show how the alimentary canal of the
arthropod became the neural canal of the vertebrate. Although this
alimentary canal is formed in the same way as before, it is no longer
recognized as homologous with the scorpion's alimentary canal, but
because it no longer absorbs pabulum, and does not therefore form
the definite gut, it is called an epiblastic tube, and, in the words of
Hay Lankester, has no developmental importance.

All the arthropods are built up on the same type, and in all the
development may in its broad outlines be referred to the type just
mentioned. So also with the vertebrate group; in both cases the
position of the central nervous system determines the starting area
of embryonic growth. In both cases the absorbing layer shows the
position of the definite gut. A concentrated nervous system of this
type is common to all the segmented animals from the annelids to
the vertebrates, and in all cases the germ-band which indicates the
first formation of the embryo is in the position of this nervous system.

As far as the embryo is concerned, there is no great difficulty in
the conception that the yolk-mass may have shifted from one side to
the other in passing from the arthropod to the vertebrate, for in the
arthropod the embryo at first is surrounded by yolk and then passes
to the periphery of the egg. If it is permissible to speak of a dorsal
and ventral surface to an egg, and we may imagine the egg held with
such dorsal surface uppermost, then the yolk would be situated
ventrally to the embryo, as in the vertebrate, if the protoplasmic
cells of the embryo rose from their central position to the surface
through the yolk, while if they sank through the yolk, the yolk
would be situated dorsally to the embryo, as in the arthropod.

THE PRINCIPLES OE EMBRYOLOGY 485

In cases where there is no yolk, or very little, as in Lucifer and
Amphioxus respectively, the embryo is compelled to feed itself at a
very early age ; such embryos form a free-swimming pelagic ciliated
bias tula, the invagination of which, for the purpose of collecting food
material out of the open sea, is the simplest method of obtaining
nutriment. Here, as in other cases, it is the physiological necessity
which determines the method of formation of the gut, and such
similarity of appearance as exists between the gastrula of Lucifer and
that of Amphioxus, by no means implies that the gut of the adult
Lucifer is homologous with the gut of Amphioxus.

I have compared two meroblastic eggs of the two classes respec-
tively, because the scorpion's egg is meroblastic. I imagine that no
real difficulty arises with respect to holoblastic eggs, for the experi-
ments of 0. Hertwig and Samassa show that by centrifugalizing,
stimulating, and breaking down of large spheres the holoblastic
amphibian egg may be converted into a meroblastic one, and then
development will proceed regularly, i.e. in this case also the growth
proceeds from the animal pole ; the large cells of the vegetal pole, like
the yolk-cells of the meroblastic egg, manufacture pabulum for the
growing syncytial host.

Summary.

Any attempt to discover how vertebrates arose from invertebrates must be
based upon the study of Comparative Anatomy, of Palaeontology, and of Embryo-
logy. The arguments and evidence put forward in the preceding* chapters
show most clearly how the theory of the origin of vertebrates from paheos-
tracans is supported by the geological evidence, by the anatomical evidence,
and by the embryological evidence. Of the three the latter is the strongest
and most conclusive, if it be taken to include the evidence given by the larval
stage of the lamprey.

The stronghold of embryology for questions of this sort is the Law of
Recapitulation, which asserts that the history of the race is recapitulated to
a greater or less extent in the development of the individual. In the previous
chapters such recapitulation has been shown for all the org-ans of the vertebrate
body. In this respect, then, embryology has proved of the g'reatest value in
continuing- the evidence of relationship between the palfeostracan and the
vertebrate, g*iven by anatomical and geological study.

There is, however, another side to embryology, which claims that the tissues
of all the Metazoa are built up on the same plan ; that in all cases in the very
early stag*e of the embryo three layers are formed, the epiblast. mesoblast, and
hypoblast ; that in all animals above the Protozoa these three layers are

486 THE ORIGIN OF VERTEBRATES

homologous, the epiblast in all eases forming the external or skin-layer, the
hypoblast the internal or gut-layer.

Such a theory, therefore, as is advocated in this book, which turns the gut
of the arthropod into the neural canal of the vertebrate, and makes a new gut
for the vertebrate from the external surface must be wrong, as it flatly
contradicts the fundamental germ-layer theory.

Of recent years grave doubts have been thrown upon the validity of this
theory, doubts which have increased in force year by year as more and more
facts have been discovered which are not in agreement with the theory. So
much is it now discredited that any criticism against my theory, which is based
upon it, weighs nothing in the balance ag-ainst the positive evidence of recapitu-
lation already stated. If the germ-layer theory is no longer credited, upon
what fundamental laws is embryology based ?

In this chapter I have ventured to suggest a reply to this question, based on
the uniformity of the laws of growth throughout the existence of the individual.

In the adult animal the body is composed of two kinds of tissues, those which
are connected with or at all events are under the control of the nervous system,
and those which are capable of leading a free life independent of the nervous
system. These two kinds of tissues can be traced back from the adult to the
embryo, and it is the task of embryology to find out how these two kinds of
tissue originate.

The following out of this line of thought leads to the conception that,
throughout the Metazoa. the body is composed of a host which consists of the
master-tissues of the body, and takes the form of a neuro-epithelial syncytium,
within the meshes of which free living independent organisms or cells live, so
to speak, a symbiotic existence.

The evidence points to the orig'in of all these free cells from germ-cells, and
thus leads to the conception that the blastula stage of every embryo represents
two kinds of cells, the one which will form the mortal host being the locomotor
neuro-epithelial cell, the other the independent immortal symbiotic germ-cell.
Such conception leads directly to the conclusion that the blastula stage of every
member of the Metazoa is the embryonic representation of a Protozoan ancestor
of the Metazoa ; an ancestor, whose nature may be illustrated by such a living
form as Volvox globator, which, like a blastula. is composed of a layer of cells
forming a hollow sphere. These cells partly bear cilia, and so form a locomotor
host, partly are of a different character, and form male and female germ-cells.
The latter leave the surface of the sphere, pass as free individuals into its
fluid contents, form spermaries and ovaries, and then by the rupture of the
mortal locomotor host pass out into the external medium, as free swimming
young* Volvox.

It is of interest to note that such members of the Protozoa are among- the
most highly developed of the members of this great group.

From such a beginning1 arose in orderly evolution, on the one hand, all the
neuro-muscular and neuro-epithelial structures of the body — the so-called master-
tissues ; on the other, the germ-cells, the blood-corpuscles, lymph-corpuscles
plasma and excretory cells, connective tissue cells, cartilage and bone-cells, etc.,
all of them independent of the central nervous system, all traceable to a
modification of the original germ-cells.

THE PRINCIPLES OF EMBRYOLOGY 487

Such a view of the processes of embryology brings embryology into harmony
with comparative anatomy and phylogeny, for it makes the central nervous
system and not the alimentary canal the most important factor in the develop-
ment of the host.

The growth of the individual, whether arthropod or vertebrate, spreads from
the position of the central nervous system, regardless of whether that position
is a ventral or dorsal one with respect to the yolk-mass. Where the pabulum
is. there is the definite gut, the lining walls of which are called in the embryo ,
hypoblast ; but when the pabulum is no longer there, although a tube is formed
in the same manner as the alimentary canal of the arthropod, it is now called
an epiblastic tube, and is known as the neural tube of the vertebrate.

This is the great fallacy of the germ-layer theory, a fallacy which consists
of an argument in a vicious circle : thus the alimentary canal is homologous in
all of the Metazoa, because it is formed of hypoblast, but there is no definition
of hypoblast, except that it is always that layer which forms the definitive
alimentary canal.

When, after the process of segmentation has been completed, a free swimming
blastula results, unprovided with any store of pabulum in the shape of yolk,
then the same physiological necessity causes such a form to obtain its nutriment
from the surrounding medium. The simplest way to do this is by a process
of invagination, in consequence of which food particles are swept into the
invaginated part and then absorbed. For this reason in such cases true
gastrulas are formed, as in the case of Amphioxus among the vertebrates, and
Lucifer among the crustaceans ; such a formation does not in the least imply
that the gut of the arthropod is homologous with that of the vertebrate. The
resemblance between the two is not a morphological one, but due to the same
physiological necessity. They are analogous formations, not homologous.

The muscular tissues are found to be formed in close connection with the
nervous tissues, and in very many cases are described as formed from epiblast,
so that there are strong reasons for placing them in a special category of the
so-called mesoblastic tissues. If they be separated out, then it seems to me, the
rest of the mesoblast would consist of the free-living cells of the body, which
are not connected with the central nervous system. In watching, then, the
formation of mesoblast, defined in this way, we are watching the separation
out from the master-tissues of the body of the independent skeletal and
excretory cells.

CHAPTER XV

FINAL REMARKS

Problems requiring investigation —

Giant nerve-cells and giant-fibres ; their comparison in fishes and in arthro-
pods ; blood- and lymph-corpuscles ; nature of the skin ; origin of system of
unstriped muscles ; orig-in of the sympathetic nervous system ; biological
test of relationship.

Criticism of Balanoglossus theory. — Theory of parallel development. — Impoi't-
ance of the theory advocated in this book for all problems of Evolution.

The discussion in the last chapter on the " Principles of Embryology "
completes the evidence which I am able to offer up to the present
time in favour of my theory of the " Origin of Vertebrates." There
are various questions which I have left untouched, but still are well
worth discussion, and may be mentioned here. The first of these is
the significance of the giant nerve-cells and giant nerve-fibres so
characteristic of the brain-region of the lower vertebrates. In most
fishes two very large cells are most conspicuous objects in any
transverse section of the 'medulla oblongata at the level of entrance
of the auditory nerves. Each of these cells gives off a number of
processes, some of which pass in the direction of the auditory nerves
and one very large axis-cylinder process which forms a giant-fibre,
known by the name of a Mauthnerian fibre. Each Mauthnerian
fibre crosses the middle line soon after its origin from the giant-cell,
and passes down the spinal cord on the opposite side right to the
tail. Here, near the end of the spinal cord, it breaks up into smaller
fibres, which are believed by Fritsch and others to pass out directly
into the ventral roots to supply the muscles of the tail. Thus Bela
Haller says : " The Mauthnerian fibres are known to give origin to
certain fibres which supply the ventral roots of the last three spinal
nerves, so that their terminal branches serve, in all probability, for
the innervation of the muscles of the tail-fin." They do not occur in
the eel, according to Haller, or in Silurus, according to Kolliker.

FINAL REMARKS 489

Their absence in those fishes, in which a well-developed tail-fin is
also absent, increases the probability of the truth of Fritsch's original
conclusion that these giant-fibres are associated axis -cylinders for
certain definite co-ordinated movements of the fish, especially for the
lateral movement of the tail.

In Ammoccetes, instead of two Mauthnerian fibres, a number of
giant-fibres are found. They are called Mullerian fibres, and arise
from giant-cells which are divisible into two groups. The first group
consists of three pairs situated headwards of the level of exit of the
trigeminal nerves. Two of these lie in front of the level of exit of
the oculomotor nerves, and one pair is situated at the same level as
the origin of the oculomotor nerves. The second group consists of
a number of cells on each side at the level of the entrance of the
fibres of the auditory nerves.

The Mullerian fibres largely decussate, as described by Ahlborn,
and then become the most anterior portion of the white matter of the
spinal cord, forming a group of about eight fibres on each side
(Fig. 73). A few fibres are also found laterally, and slightly
dorsally, to the grey matter. These giant-fibres pass down the spinal
cord right to the anal region ; their ultimate destination is unknown.
Mayer considers that in the first part of their course they correspond
to those tracts of fibres known as the " posterior longitudinal bundles "
in other vertebrates. I imagine, therefore, that the spinal part of their
course represents the two antero-lateral descending tracts. The
second group of giant-cells, which appears to have some connection
with the auditory nerves, may represent " Deiter's nucleus." The
whole system is probably the central nervous part of a co-ordination
mechanism, which arises entirely in the pro-otic or prosomatic region
of the brain — the great co-ordinating and equilibrating region par
excellence.

If we turn now to the arthropod it is a striking coincidence that
in the crayfish and in the lobster the work of Eetzius, of Celesia,
of Allen, and of many others demonstrates the existence of an
equilibration-mechanism for the swimming movements of the tail-
muscles, which is carried out by means of giant-fibres. These giant-
fibres are the axis-cylinder processes of giant-cells, situated exclusively
in the brain-region, and they run through the whole ventral ganglionic
chain in order to supply the muscles of the tail. In the ventral
nerve-cord of the crayfish, according to Eetzius, two specially large

490 THE ORIGIN OF VERTEBRATES

giant-fibres exist, each of which breaks up, at the last abdominal
ganglion, into smaller fibres, which pass directly out with the nerves
to the tail-fin. Allen has shown that, in addition to these two
specially large giant-fibres, there are a number of others, some of
which, similarly to the Mvillerian fibres of Ammocoetes, cross the
middle line, while some do not. Each of these arises from a large
nerve-cell and passes to one or other of the last pair of abdominal
ganglia. The latter fibres, he says, send off collaterals, while the
two specially large giant-fibres do not. The cells which give origin
to all these large, long fibres are situated in or in front of the proso-
matic region of the brain, similarly to the giant-cells, which give rise
to the corresponding Mullerian fibres of Ammocoetes. I do not know
how far this system is represented in Limulus or Scorpio.

It is, to my mind, improbable that theMauthnerian fibres pass out
directly as motor fibres to the muscles of the tail-fin ; it is more
likely that they are conducting paths between the equilibration-
mechanism in connection with the Vlllth nerve and the spinal
centres for the movements of the tail. Similarly, with respect to
the arthropod, it is difficult to believe that the motor fibres for the
tail-muscles arise in the brain-region. In either case, the striking
coincidence remains that the movements of the tail-end of the body
are regulated by means of giant-fibres which arise from giant-cells in
the head-region of the body in both the Arthropoda and the lowest
members of the Vertebrata.

The meaning of this system of giant-cells and giant-fibres in both
classes of animals is well worthy of further investigation.

Another important piece of comparative work which ought to
help in the elucidation of this problem is the comparison of the blood-
and lymph-corpuscles of the vertebrate with those of the invertebrate
groups. As yet, I have not myself made any observations in this
direction, and feel that it is inadvisable to discuss the results of
others until I know more about the facts from personal observation.

The large and important question of the manner of formation of
the vertebrate skin has only been considered to a slight extent.
A much more thorough investigation requires to be made into the
nature of the skin of the oldest fishes in comparison with the skin of
Ammoccetes on the one side, and of Limulus and the Pakeostraca
on the other.

The muscular system requires further investigation, not so much

FINAL REMARKS 49 1

the different systems of the striated voluntary musculature — for these
have been for the most part compared in the two groups of animals
in previous chapters — as the involuntary unstriped musculature,
about which no word has been said. The origin of the different
systems of unstriped muscles in the vertebrate is bound up with
the origin of the sympathetic system and its relation to the cranial
and sacral visceral systems. The reason why I have not included in
this book the consideration of the sympathetic nervous system is on
account of the difficulty in finding any such system in Aminocoetes.
Also, so far as I know, the distribution of unstriped muscle in
Ammocoetes has not been worked out.

One clue has arisen quite recently which is of great importance,
and must be worked out in the future, viz. the extraordinary con-
nection which exists between the action of the sympathetic nervous
system and the action of adrenalin. This substance, which is
obtained from the medullary part of the adrenal or suprarenal glands,
when injected into an animal produces the same effects as stimulation
of the nerves, which belong to the lumbo-thoracic outflow of visceral
nerves, i.e. the system known as the sympathetic nervous system,
which is distinct from both the cranial and sacral outflows of visceral
nerves. The similarity of its action to stimulation of nerves is
entirely confined to the nerves of this sympathetic system, and never
resembles that of either the cranial or sacral visceral nerves.

Another most striking fact which confirms the great importance
of this connection between the adrenals and the sympathetic nervous
system from the point of view of the evolution of the latter system is
that the extract of the adrenals always produces the same effect
as that of stimulation of the nerves of the sympathetic system,
whatever may be the animal from which the extract is obtained.
Thus adrenalin obtained from the elasmobranch fishes will produce
in the highest mammal all the effects known to occur upon stimula-
tion of the nerves of its sympathetic system.

Further, the cells, which are always associated with the presence
of this peculiar substance — adrenalin — stain in a characteristic manner
in the presence of chromic salts. In Ammocoetes patches of cells
which stain in this manner have been described in connection with
blood-vessels in certain parts, so that, although I know of no definite
evidence of the existence of cell-groups in Ammocoetes corresponding
to the ganglia of the sympathetic system in other vertebrates, it is

492 THE ORIGIN OF VERTEBRATES

possible that further investigation into the nature and connection of
these " chromaffine " cells may afford a clue to the origin of the
sympathetic nervous system. At present it is premature to discuss
the question further.

Finally, another test as to the kinship of two animals of different
species must be considered more fully than I have been able to do
up to the present time. This test is of a totally different nature to
any put forth in previous pages. It is known as the " biological
test " of relationship, and is the outcome of pathological rather than
of physiological or anatomical research. It is possible that this test
may prove the most valuable of all. At present we do not know
sufficiently its limitations and its sources of error, especially in the
case of cold-blooded animals, to be able to look upon it as decisive in
a problem of the kind considered in this book.

The nature of this test is as follows : It has been found that the
serum of the blood of another animal, when injected in sufficient
quantity into a rabbit, will cause such a change in the serum of that
rabbit's blood that when it is added to the serum of the other animal
a copious precipitate is formed, although the serum of normal rabbit's
blood when mixed with that of another animal will cause no precipi-
tate whatever. This extraordinary production of a precipitate in the
one case and not in the other indicates the production of some new
substance in the rabbit's serum in consequence of the introduction of
the foreign serum into the rabbit, which brings about a precipitate
when the rabbit's serum containing it is mixed with the serum
originally injected. The barbarous name "antibody " has been used
to express this supposed substance in accordance with the meaning
of such a word as " antitoxin," which has been a long time in use in
connection with preventive remedies against pathogenic bacteria.
Now, it is found that the rabbit's serum containing a particular
" antibody " will cause a precipitate only when added to the serum
of the blood of the animal from which the " antibody " was produced
or to the serum of the blood of a nearly related animal.

Further, if that animal is closely related a precipitate will be
formed nearly as copious as with the original serum, if more distantly
related a cloudiness will occur rather than a precipitate, and if the
relationship is still more distant the mixture of the two sera will
remain absolutely clear. Thus this test demonstrates the close
relationship of man to the anthropoid apes and his more distant

FINAL REMARKS 493

relationship to monkeys in general. By this method very evident
blood-relationships have been demonstrated, especially between
members of the Mammalia.

I therefore started upon an investigation into the possibility of
proving relationship in this way between Limulus and Aminoccetes,
with the kind assistance of Mr. Graham Smith. I must confess I
was not sanguine of success, as I thought the distance between
Limulus and Ammoccetes was too great. Dr. Lee, of New York,
kindly provided me with most excellent serum of Limulus, and
the tirst experiments showed that the anti-serum of Limulus gave a
most powerful precipitate with its own serum. Graham Smith then
tried this anti-serum of Limulus with the serum of Ammoccetes, and
to his surprise, and mine, he obtained a distinct cloudiness, indicative
of a relationship between the two animals. This, however, is not
considered sufficient, the reverse experiment must also succeed. I
therefore, with Graham Smith, obtained a considerable amount of
blood from the adult lampreys at Brandon, and produced an anti-
serum of Petromyzon, which gave some precipitate with its own
serum, but not a very powerful one. This anti-serum tried with
Limulus gave no result whatever, but at the same time it gave no
result with serum from Ammoccetes, so that the experiment not only
showed that Petromyzon was not related to Limulus, but also was
not related to its own larval form, which is absurd.

Considerable difficulties were encountered in preparing the
Petromyzon anti-serum owing to the extreme toxic character of the
lamprey's serum to the rabbit ; in this respect it resembled that of
the eel. It is possible that the failure of the lamprey's anti-serum
was due to the necessity of heating the serum sufficiently to do
away with its toxicity before injecting it into the rabbit. At this
point the experiments have been at present left. It will require a
long and careful investigation before it is possible to speak decisively
one way or the other. At present the experiment is positive to a
certain extent, and also negative ; but the latter proves too much, for
it proves that the larva is not related to the adult.

Some day I hope this " biological test " will be of use for
determining the relationships of the Tunicata, the Enteropneusta,
Amphioxus, etc., as well as of Limulus and Ammoccetes.

The origin of Vertebrates from a Pakeostracan stock, as put
forward in this book, gives no indication of the systematic position

494 THE ORIGIN OF VERTEBRATES

of the Tunicata or Enteropneusta. Neither the Tunicata nor
Ainphioxus can by any possibility be on the direct line of ascent
from the invertebrate to the vertebrate. They must both be looked
upon as persistent failures, relics of the time when the great change
to the vertebrate took place. The Enteropneusta are on a different
footing; in their case any evideuce of affinity with vertebrates is
very much more doubtful.

The observer Spengel, who has made the most exhaustive study
of these strange forms, rejects in toto any connection with vertebrates,
and considers them rather as aberrant annelids. The so-called
evidence of the tubular central nervous system is worth nothing.
There is not the slightest sign of any tubular nervous system in the
least resembling that of the vertebrate. It is simply that in one place
of the collar-region the piece of skin containing the dorsal nerve of
the animal, owing to the formation of the collar, is folded, and thus
forms just at this region a short tube. My theory explains in a
natural manner every portion of the elaborate and complicated tube
of the vertebrate central nervous system. In the Balanoglossus
theory the evolution of the vertebrate tube in all its details from this
collar-fold is simple guesswork, without any reasonable standpoint.
Similarly, the small closed diverticulum of the gut in Balanoglossus,
which is dignified with the name of " notochord," has no right to the
name. As I have already said, it may help to understand why the
notochord has such a peculiar structure, but it gives no help to
understanding the peculiar position of the notochord. The only
really striking resemblance is between the gill-slits of Amphioxus
and of the Enteropneusta. In this comparison there is a very great
difficulty, very similar to that of the original attempts to derive
vertebrates from annelids — the gill-slits open ventrally in the one
animal and dorsally in the other. In both animals an atrial cavity
exists which is formed by pleural folds, and in these pleural folds
the gonads are situated so that the similarity of the two branchial
chambers seems at first sight very complete. In the Enteropneusta,
however, there are certain forms — Ptychodera — in which these pleural
folds have not met in the mid-line in this branchial region, and in
these it is plainly visible that these folds, with their gonads, spring
from the ventral mid-line and arch over the dorsal region of the
body. Equally clearly Amphioxus shows that its pleural folds,
with the gonads, spring from the dorsal side of the animal,

FINAL REMARKS

495

and grow ventral wards until they fuse in the ventral mid-line (cfm
Fig. 168).

As far, then, as this one single striking similarity between
Amphioxus and the Enteropneusta is concerned it necessitates the
reversal of dorsal and ventral surfaces to bring the two branchial
chambers into harmony.

In a mud-dwelling animal, like Balanoglossus, which possesses no
appendages, no special sense-organs, it seems likely enough that
ventral and dorsal may be terms of no particular meaning, and con-
sequently what is called ventral in Balanoglossus may correspond to
what is dorsal in Amphioxus ; in this way the branchial regions of the

CN.S

V.A

V.A.

Fig. 168. -Diagram illustrating the Position of the Pleural Folds and
Gonads in Ptychodera (A) and Amphioxus (B) respectively.

Al., alimentary canal ; D.A., dorsal vessel; V.A., ventral vessel; cj., gonads; NC,
notochord ; C.N.S., central nervous system.

two animals may be closely compared. Such comparison, however,
immediately upsets the whole argument of the vertebrate nature
of Balanoglossus based on the relative position of the central nervous
system and gut, for now that part of its nervous system which is
looked upon as the central nervous system in Balanoglossus is ventral
to the gut, just as in a worm-like animal, and not dorsal to it as
in a vertebrate.

There is absolutely no possibility whatever of making such a
detailed comparison between Balanoglossus and any vertebrate, as
I have done between a particular kind of arthropod and Ammoccetes.
In the latter case not only the topographical anatomy of the organs
in the two animals is the same, but the comparison is valid even to
microscopical structure. In the former case the origin of almost all

496 THE ORIGIN OF VERTEBRATES

the vertebrate organs is absolutely hypothetical, no clue is given in
Balanoglossus, not even to the segmented nature of the vertebrate.
The same holds good with the evidence from Embryology and from
Palaeontology. I have pointed out how strongly the evidence in both
cases confirms that of Comparative Anatomy. In neither case is the
strength of the evidence for Balanoglossus in the slightest degree
comparable. In Embryology an attempt has been made to compare
the origin of the ccelom in Amphioxus and in Balanoglossus. In
Palaeontology there is nothing, only an assumption that in the
Cambrian and Lower Silurian times a whole series of animals were
evolved between Balanoglossus and the earliest armoured fishes, which
have left no trace, although they were able to hold their own against
the dominant Palaeostracan race. The strangeness of this conception
is that, when they do appear, they are fully armoured, as in Pteraspis
and Cephalaspis, and it is extremely hard luck for the believers in
the Balanoglossus theory that no intermediate less armoured forms
have been found, especially in consideration of the fact that the
theory of the origin from the Palaeostracan does not require such
intermediate forms, but finds that those already discovered exactly
fulfil its requirements.

One difficulty in the way of accepting the theory which I have
advocated is perhaps the existence of the Tunicata. I cannot see
that they show any affinities to the Arthropoda, and yet they are
looked upon as allied to the Vertebrata. I can only conclude that
both they and Amphioxus arose late, after the vertebrate stock had
become well established, so that in their degenerated condition they
"ive indications of their vertebrate ancestry and not of their more
remote arthropod ancestry.

In conclusion, the way in which vertebrates arose on the earth as
suggested in this book carries with it many important far-reaching
conclusions with respect to the whole problem of Evolution.

When the study of Embryology began, great hopes were entertained
that by its means it would be possible to discover the pedigree of
every group of animals, and for this end all the stages of development
in all groups of animals were sought for and, as far as possible,
studied. It was soon found, however, that the interpretation of
what was seen was so difficult, as to give rise to all manner of views,
depending upon the idiosyncracy of the observer. At his will he
decided whether any appearance was cuenogenetic or palingenetic,

FINAL REMARKS 497

with the result that, in the minds of many, embryology has failed to
afford the desired clue.

At the same time, the geological record was looked upon as too
imperfect to afford any real help ; it was said, and is said, that the
Cambrian and pre-Cambrian periods were so immense, and the animals
discovered in the lower Silurian so highly organized, as to compel
us to ascribe the origination of all the present-day groups to this
immense early period, the animals of which have left no trace of
their existence as fossils.

In consequence of, or at all events following upon, the supposed
failure of embryology and of geology to solve the problem of the
sequence of evolution of animal life, a new theory has arisen, which
goes very near to the denial of evolution altogether. This is the
theory of parallel development. It discards the old picture of a genealo-
gical tree with main branches arising at different heights, these again
branching and branching into smaller and smaller twigs, and substitutes
instead the picture of the ribs of a fan, every rib running independently
of every other, each group represented by a rib reaching its highest
development on the circumference of the fan and coming nearer
and nearer to a common point at the handle of the fan. This point
of convergence, where all the groups ultimately meet, is so far back
as to reach'to the lowest living organisms.

This, in my opinion, unscientific and inconceivable suggestion has
arisen largely in consequence of a conception which has become
firmly fixed in the minds of very many writers on this subject — the
conception that in the evolution of every group, the higher members
of the group are the most specialized in the peculiarities of that group,
and it is impossible to obtain a new group with different peculiarities
from such specialized members. If, then, a higher group is to arise
from a lower, it must arise from the generalized members of that
lower group, in other words, from the lowest members or those
nearly akin to the next lower group.

Similarly, the highest members of this latter group are too
specialized, and again we must go to the more generalized members
of the group. In this way each separate specialized group is put-on
one side, and so the conception of parallel development comes into
being.

The evidence given in this book dealing with the origin of
vertebrates strikes at the foundations of this belief, for it presents an

2 K

49§ THE ORIGIN OF VERTEBRATES

image of the sequence of evolution of animal forms in orderly upward
progress, caused by the struggle for existence among the members of
the race dominant at the time, which brought about the origin of the
next higher group not from the lowest members of the dominant
group, but from some one of the higher members of that group.

The great factor in evolution has been throughout the growth of
the central nervous system ; from that group of animals which
possessed the highest nervous system evolved up to that time the
next higher group must have arisen.

In this way we can trace without a break, always following out
the same law, the evolution of man from the mammal, the mammal
from the reptile, the reptile from the amphibian, the amphibian
from the fish, the fish from the arthropod, the arthropod from
the annelid, and we may be hopeful that the same law will enable
us to arrange in orderly sequence all the groups in the animal
kingdom.

This very same law of the paramount importance of the develop-
ment of the central nervous system for all upward progress will, I
firmly believe, lead to the establishment of a new and more fruitful
embryology, the leading feature of which will be, as suggested in the
last chapter, not the attempt to derive from the blastula three germ-
layers common to all animals, but rather two sets of organs — those
which are governed by the nervous system and those which are not —
and thus by means of the development of the central nervous system
obtain from embryology surer indications of relationship than are
given at present.

The great law of recapitulation, which asserts that the past
history of the race is indicated more or less in the development of
each individual, a law which of late years has fallen somewhat into
disrepute, owing especially to the difficulty of interpreting the
embryological history of the vertebrate, is triumphantly vindicated
by the theory put forward in this book. Each separate vertebrate
organ, one after the other, as shown in the last chapter, indicates in its
development the manner in which it arose from the corresponding-
organ of the arthropod. There is no failure in the evidence of
embryology, the failure is in the interpretation thereof.

So, too, my theory vindicates the geological method. There is no
failure here ; on the contrary, the record of the rocks proclaims with
startling clearness not only the sequence of evolution in the

FINAL REMARKS 499

vertebrate kingdom itself, but the origin of the vertebrate from the
most highly-developed invertebrate race.

The study of the comparative anatomy of organs down to the
finest details has always been a most important aid in finding out
relationship between animals or groups of animals. My theory
endorses this view to the uttermost, and especially indicates the
study of the central nervous system and its outgoing nerves as that
comparative study which is most likely to afford valuable results.

As for the individual, so for the nation ; as for the nation, so for
the race ; the law of evolution teaches that in all cases brain-power
wins. Throughout, from the dawn of animal life up to the present
day, the evidence given in this book suggests that the same law
has always held. In all cases, upward progress is associated with
a development of the central nervous system.

The law for the whole animal kingdom is the same as for the
individual. " Success in this world depends upon brains."

BIBLIOGRAPHY AND INDEX OF

AUTHORS

Author's name.

AHLBORN

AICHEL .
ALCOCK .

ALLEN . . . ,
ANDERSON, H. K.

APATHY . . .

ASSHETON

Title of Paper.

Pages of
reference.

" Untersuchungen iiber das Gehirn der Pe-

tromyzonten "

Zeitsch. f. wiss. Zool. Vol. 39. 1883
" Leber die Segmentation des Wirbelthier-

korpers "

Zeitsch. f. wiss. Zool. Vol. 40. 1884
" Vergleicbende Entwicklungsgeschichte und
Stanmiesgeschichte der Nebennieren " .

Arch. f. Mikr. Anat. Vol. 56. 1900

" The Peripheral Distribution of the Cranial

Nerves of Ammoccetes "

Journ. of Anat. and Physiol. Vol. 33. 1898

" On Proteid Digestion in Ammoccetes " .
Journ. of Anat. and Physiol. Vol. 33. 1898
" Studies on the Nervous System of Crus-

Q.j'.Micr.'Sci Vol! 36. 1894

" The Nature of the Lesions which hinder the
Development of Nerve-cells and their Pro-
cesses "

Journ. of Physiol. Vol. 28. 1902
" On the Myelination of Nerve-fibres "

Report of the Brit. Assn. 1898

"Das leitende Element des Nervensystems

und seine topographischen Beziehung zu

denZellen"

Mitth. a. d. Zool. Stat, zu Neapel. Vol. 12.

1896
"On the Phenomenon of the Fusion of the
Epiblastic Layers in the Rabbit and in the

Frog"

Q. J. Micr. Sci. Vol. 37. 1894

"An Experimental Examination into the

Growth of the Blastoderm of the Chick " .

Proc. of Roy. Soc. Vol. 60. 1896

210, 489
260

424, 428

135, 287, 288,
289, 304, 307,
347, 445

164,171, 177,

188, 202, 297,

300, 310, 311,

316

58, 213, 442,

452

489

448, 470

466, 467, 469

467, 477

467

42
154

502

THE ORIGIN OF VERTEBRATES

Author's name.

ASSHETON

BALFOUR

BARKER

BATESON

BEARD .

Title of Paper.

BECK and LAN-
KESTER

BEECHER . . .

BELL, C. . . .

BELLONCI . . .

Length

of the Frog

Pages of
reference.

" On the Growth in
Embryo"

Q. J. Micr. Sci. Vol. 37. 1894

" A Re-investigation into the Early Stages of

the Development of the Rabbit " ...

Q. J. Micr. Sci. Vol. 37. 1894

"The Primitive Streak of the Rabbit: the

Causes which may determine its Shape, and

the part of the Embryo formed by its

Activity"

Q. J. Micr. Sci. Vol. 37. 1894

' Comparative Embryology.' Vol. 2 .

London. 1881. Macmillan & Co.

" On the Origin and History of the Urino-

genital Organs of Vertebrates " .

Joum. of Anat. and Physiol. Vol. 10. 1876

" On the Nature of the Organ in Adult

Teleosteans and Ganoids, which is usually

regarded as the Head-kidney or Pronephros "

Q. J. Micr. Sci. Vol. 22. 1882

' The Nervous System '

London. 1901

" The Ancestry of the Chordata " ....

Q. J. Micr. Sci. Vol. 26. 1886

' Materials for the Study of Variation ' . .

London. 1894

"The System of Branchial Sense Organs and

their Associated Ganglia in Ichthyopsida "

Q. J. Micr. Sci. Vol. 26. 1S85

" The Development of the Peripheral Nervous

System in Vertebrates "

Q. J. Micr. Sci. Vol. 29. 1888
"The Old Mouth and the New" ....

Anat. Anzciger. 1888
" The Source of Leucocytes and the True

Function of the Thymus "

Anat. Anzciger. Vol. 18. 1900

" The Parietal Eye of the Cyclostome Fishes "

Q. 'j. Micr. Sci. Vol. 29. 1882

" On the Muscular and Endo-skeletal Tissues

of Scorpio "

Trans. Zool. Soc. Vol. 11. 1885

" Natural Classification of the Trilobites " .

Amer. Joum. of Sci. Ser. 4. Vol. 3. 1897

' The Nervous System of the Human Body ' .

London. 1830
" Systeme Nerveux et Organes des sens du

Spharoma scrratum"

Archiv. Ital. de Biol. Vol. 1. 1882

BENHAM and LAN-
KESTER

les rapports des lobes
Arthropods superieurs

Sur la structure et
olfactives dans les
et les Vertebres " ,

Archiv. Ital. dc Biol. Vol. 3. 1883
1 On the Muscular and Endo-skeletal Systems

of Limulus " .

Trans. Zool. Soc. Vol. 11. 1885

154
154

154

73, 74, 94,
103, 104, 120,
181, 259, 424

390, 392

420
470
11

387

262, 281, 283

262, 281, 283
318

425, 426

171,222,224,

247, 268-277

283, 351, 436,

437

155, 156, 183

62, 90, 92,
101

221, 225

143,171,176,
177, 247

BIBLIOGRAPHY AND INDEX OF AUTHORS

503

Author's name.

Title of Paper.

Pages of
reference.

BERGER . . .

" Untersuchungen iiber den Bau des Gehirns

und der Retina der Arthropoden "...

88-92, 97

Arbeit, a. d. Zool. Instit. Wien. Vol. 1. 1878

100, 101

BERGH ....

" Neue Beitrage zur Embryologie der Anne-

478

Zeitsch. f. wiss. Zool. Vol. £0. 1890

BERKLEY . . .

" The Intrinsic Nerves of the Kidney "
Bulletin of the Johns Hopkins Hospital. Vol. 4

477

BERNARD . . .

' The Apodidse : a Morphological Study ' .

Nature Series. 1892

284

BERTKAU . . .

" Beitrage zur Kenntniss der Sinnesorgane

der Spinnen. 1. Die Augen der Spinnen "

369

Archiv. f. mikr. Anat. Vol. 27. 1886

BIEDERMANN .

Translated by P. A. Welby. London. 1896

BLANCHARD . .

225

' L' Organisation du Regne Animal. Arachnides '

109, 177,

190,

Paris. 1852

206, 313,

315

BLES

" The Correlated Distribution of Abdominal

Pores and Nephrostomes in Fishes "

431

Joum. of Anat. and Physiol. Vol. 32. 1898

BOBRETSKY . .

' Development of Astacus and Palsemon ' .

Kiew. 1873

BOURNE and LAN-

See Lankester and Bourne.

KESTER

BOVERI ....

" Die Nieren Canalchen des Amphioxus " .

392, 395,

402,

Zool. Jahrbuch. Vol. 5. 1892

407, 412,

427

426,

BRAEM ....

Biol. Centralblatt. Vol. 15. 1895

460, 461

462

BRAUER. . . .

"Beitrage zur Kenntniss der Entwicklungs-

geschichte des Skorpions "

62, 167,

222,

Zeit. f. wiss. Zool. Part I. Vol. 57. 1894

237, 281,

482

Part II. Vol. 59. 1895

" Beitrage zur Kenntniss der Entwicklung

und Anatomie der Gymnophionen." III.

" Die Entwicklung der Excretionsorgane " .

393, 394,

400,

Zool. Jahrbuch. Vol. 16. 1902

402

" Ueber die Entwicklung von Hydra " . .

473

Zeit. f. wiss. Zool. Vol. 52. 1891

BUTSCHLI . . .

"Notiz zur Morphologic des Auges der Mu-

114

Festschrift des Natur-hist-med. Vereins zu

Heidelberg. 1886

BUJOR ....

" Contribution a l'etude de la metamorphose
de V Ammoccetes branchialis en Petromyzon

135, 304

Revue Biologique du Nord de la France.

Vol. 3. 1891

177, 315,

316

CELESIA . . .

' Differenziamento della proprieta mibitoria

e dei funzioni coordinatrici nella catena

gangliare dei crustacei decapodi ' ...

489

Genoa. 1897

CLAUS ....

" Untersuchungen iiber den Organismus und

Entwicklung von Branchipus und Artemia "

90-92,97

,100

Arbeit a.d. Zool. Institut. Wien. Vol. 6. 1886

396

504

THE ORIGIN OF VERTEBRATES

Author's name.

COPE . . . .
CRONEBERG .

CUENOT . .

CUNNINGHAM,
J. T.

DANA . . . .
DEAN-BASHFORD

DENDY

DIETL
DOHRN

DREVERMANN

EDGEWORTH .

EDINGER . .
v. EICHWALD .

Pages of
reference.

" On the Phylogeny of the Vertebra ta " . . 343

Proc. Amer. Philos. Soc, Vol. 30. 1892
" Ueber die Mumdtbeile der Aracbniden "

/ Archiv. f. Naturgeschichte. 1880

"Etudes sur le sang et les glandes lyrnpha-

tiques dans la serie animale ; 2nd partie ;

invertebres"

Arch, d, Zool. exper. gen. 2nd Ser. Vol. 9. 1891

" Tbe Significance of Kupffer's Vesicle, with

Remarks on other Questions of Vertebrate

Morphology" 318

Q. J. Micr. Sci. Vol. 25. 1885
" The Nephridia of Lanice conchilega^ . . 403
Nature. Vol. 36. 1887

" On Cephalization " 53

Mag. of Nat. Hist, 1863

' Fishes, Living and Fossil ' 344

New York. 1895

" On the Embryology of Bdellostoma Stouti" 405

Festschr. z. siebenzigsten Geburtstag. von

C. v. Kupffer. Jena. 1899

" On the Parietal Sense-organs and Associated

Structures in the New Zealand Lamprey

(Gcotria australis) " 80, 82

Q. J. Micr. Sci. Vol. 51. 1907
" Die Organisation des Arthropoden Gehirus " 101

Zeitsch. f. wiss. Zool. Vol. 27. 1876
' Der Ursprung der Wirbelthiere und das

Princip des Functions Wechsels ' ... 14, 60, 185,

Leipzig. 1875 i 186, 317, 318

Studien zur Urgeschichte des Wirbelthiere

Korpers. VIII. "Die Thyroidea bei Pe-

tromyzon, Amphioxus, und Tunicaten" . 188,195-198,

Mitth. Zool. Stat. z. Neapcl. Vol. 6. 1886 199, 212, 213

" Neue Grundlagen zur Beurtheilung der

Metamerie des Kopfes " 262,263,279

Mitth. Zool. Stat, z. Neapel. Vol. 9. 1890

Studien zur Urgeschichte des Wirbelthiere

Korpers. XIII. " Ueber Nerven und Gefiisse

bei Ammoccetes und Petromyzon Planeri ". 167, 314, 337

Mitth. Zool. Stat. z. Neapel. Vol. 8. 1888

" Ueber Pteraspis dunensis " 29,30

Zeitschr. d. Deutsch. Geol. Gesellschaft.

Vol. 56. 1904

" The Development of the Head-muscles in
Gallus domesticus, and the Morphology of
the Head-muscles in the Sauropsida " . . 266
Q. J. Micr. Sci. Vol. 51. 1907

' Anatomy of Central Nervous System in Man

and in Vertebrates' 17,264

Translated by Hall. 1899

" Die Thior-und Pflanzenreste des alten rothen
Sandsteins und Bergkalks im Nowgorod-
schen Gouvernement " 327

Bull. Sci. de VAcad. Impir. d. St. Pcters-
bourg. 1840

BIBLIOGRAPHY AND INDEX OF AUTHORS

505

Author's name.

EISIG

ELLIOTT

EMERY .

FOSTER, M.
FREUND

FRITSCH, G.
FRORIEP .

FURBRINGER, M.

GAUBERT

GEGENBAUR

v. GEHUCHTEN

GOETHE
GOTTE .

Title of Paper.

Pages of
reference.

"Die Seiten - orgaue und becherfdrmigea
Organe der Capitelliden "

Mitth. a. d. Zool. Stat. z. Neapel. Vol. 1. 1879
"Capitelliden"

Faun. u. Flor. d. Golfcs v. Neapel. Vol. 16. 1887

" On the Innervation of the Ileo - colic

Sphincter "

Journ. of Physiol. Vol.31. 1904

Quoted by Weldon

Text-book of Physiology

" Die Beziehungen der Schilddriise zu den

weiblichen Geschlechtsorganen " ...

Deutsch, Zeitsch. f. Chirugie. Vol. 18. 1883

' Untersuchungen iiber den feineren Bau des

Fischgehirns '

GOLGI .
GOODRICH

Berlin. 1878
" Ueber Anlagen von Sinnesorganen am Faci-
alis, Glossopharyngeus und Vagus, viber die
genetische Stellung des Vagus zuni Hypo-
glossus, und viber die Herkunft der Zungen-

musculatur"

Arch. f. Anat. u. Physiol; Anat. Abtheil. 1835

' Ueber die Spino-occipetalen Nerven der

Selachier und Holocephalen ' ....

Fest-schrift fiir Carl Gegenbaur. 1897

' Recherches sur les organes des sens et sur
les systemes tegumentaire, glandulaire et
rnusculaire des appendices des arachnides '

Paris. 1892
" Anatornische Untersuchung eines Limulus "
Abhandl. d. Naturforsdi. Gesellsch. z. Halle.

Vol. 4. 1858

" Ueber die Skeletgewebe der Cyclostomen " .

Jen. Zeitschrift. Vol. 5. 1870

Untersuchungen zur vergleichende Anatomie

der Wirbelthiere III. Heft. 'Das Kopf-

skelet der Selachiern '

Leipzig. 1872
' Grundriss der vergleichenden Anatomie '

Leipzig. 1878
" De l'origine du pathetique et de la racine

superieure du trijumeau "

Acad. d. Sci. Belg. Bulletin. 3rd Ser. Vol. 29.
1895

' Entwicklungsgeschichte der Unke ' . . .

Leipzig. 1875

357
357

449
420
108
215

488, 489

"On the Structure of the Excretory Organs

of Amphioxus "

Q. J. Micr. Sci. Vol. 45. 1902
" On the Nephridia of the Polychceta."

Parts I., II., Ill

Q. J. Micr. Sci. Vols. 40, 41, 43

" On the Excretorv Organs of Amphioxus " .

Proc. Roy. Soc. Vol. 69. 1902

261,262,281,
283

276-278, 409

364, 368-375
20, 358-360

181

151, 259, 261
392

264

258

101, 102, 114

72, 465, 477

395, 396, 477

395

477

506

THE ORIGIN OF VERTEBRATES

Author's name.

GRABER .
GRENACHER
GUDDEN .

HA ECKEL . .
HALLER, BELA

HARDY

HARDY and MAC-
DOUGALL . .

HATSCHEK

HAZEN ....
HEIDENHAIN
HEIDER . . .
HENSEN . . .

HENSEN and

VOELCKERS .
HERTWIG, 0., and

SAMASSA . . .

HIS

HOFFMANN

HOLM

HOYER

Title of Paper.

Pages of
reference.

" Die Chordo-tonalem Sirmesorgane und das

Gehor der Insecten " ' 364, 369-371

Archiv. f. Mikr. Anat. Vols. 20 and 21. 1882
' Untersuchungen iiber das Sehorgan der j

Arthropoden ' | 76, 100

Gottingen. 1879
Quoted in Obersteiner 264

" Untersuchungen iiber die Hypophyse und

die Infundibuliirorgane "

Morph. Jahrbuch. Vol. 25. 1898
" Untersucbungen iiber das Riickenrnark der

Teleostier"

Morph. Jahrbuch. Vol. 23. 1895
" On tbe Histological Features and Physio-
logical Properties of the Post-cesopbageal
Nerve-cord of tbe Crustacea " . . . .
Phil. Trans. Boy. Soc. 1894. B.

" On tbe Structure and Functions of tbe

Alimentary Canal of Dapbnia " . . . .

Proc. Camb. Phil. Soc. Vol. 8. 1893

" Die Metamerie des Ampbioxus und des
Amrnoccetes

Anat. Anzeig., 7 Jabrgang, 1892. Ycrhandl.
d. Anat. Gesell. in Wien, p. 136

" Studien iiber Entwicklung des Ampbioxus "
Arbeit, d. Zool. Inst. z. Wien, Vol. 4. 1881

Quoted by Lankester

See Patten and Hazen.

See Korscbelt and Heider.
" Zur Entwicklung des Nervensystem " . .
Virchows Archiv. Vol. 30. 1864

Archiv. f. Opthalmol. Vol. 24. 1878 . . .

Quoted in Zeigler's ' Lebrbucb der vergleicben-
den Entwicklungsgeschichte der niederen
Wirbeltbiere.' 1902

" Die Neuroblasten und deren Entstebung im
embryonalen Mark "

Archiv. f. Anat, u. Physiol. Anat. AbtJi.

1889

" Ueber die Metamerie des Nacbbirns und
Hinterbirns, und ibre Beziebung zu den
segmentalen Kopfnerven bei Reptilien

embryonen "

Zool. Anzeiger. Vol. 12. 1889

" Ueber die Organisation des Eurypterus
Fischcri"

Mem, d. VAcad. Imp. d. Sci. d. St. Pctersbourg.
Vol. 8. 1898

" Ueber den Nachweis des Mucins in Geweben

Mittelst der Farbe-Metbode " ....

Archiv. f. Mikr. Anat. Vol.36. 1890

461, 462
320, 321

488
110, 159

112, 206

289, 300,

337

407

475

258, 259

465, 466

265, 266

485
465, 466

276

192, 240, 241,
306

131

BIBLIOGRAPHY AND INDEX OF AUTHORS

507

Author's name.

HUXLEY

JAEKEL

Title of Paper.

JOHNSON

JOSEPH ....

JULIN and VAN
BENEDEN . .

KAENSCHE . .
v. KENNEL . .

KERR ....

KILLIAN . . .
KISHINOUYE . .

KLEINENBERG .
v. KOLLIKER . .

V. KOLLIKER and
TERTERJANZ .

KOHL ....

KOHN

" Hunterian Lectures." 1869

" On the Structure of the Mouth and Pharynx

of the Scorpion"

Q. J. Micr. Sci. Vol. 8. 1860
" On the Anatomy and Affinities of the Genus

Pterygotus "

Mem. of the Geol. Survey. Monograph I. 1859

" On Cephalaspis and Pteraspis " . . . .

Q. J. of Geol. Soc. Vol. 14. 1858

" Ueber Tremataspis und Patten's Ableitung

der Wirhelthiere von Arthropoden "
Protocoll der Deutschen Geolog. Gesellschaft,

p. 84 ; in Zeitsch. d. Deutschen Gcologischen

Gesellsch. Vol. 55. 1903
" Ueber die Organisation und systematische

Stellung der Asterolepiden "

Ibid., p. 41
" Contributions to the Comparative Anatomy

of the Mammalian Eye, chiefly based on

Opthalmoscopic Examination ....

Phil. Trans. Boij. Soc. B. Vol. 194. 1901

" Ueber das Achsenskelett des Amphioxus" .

Zeitsch. f. wiss. Zool. Vol. 59. 1895

Recherches sur l'Organisation des Ascidies
simples. " Sur l'hypophyse," etc. .

Archives de Biologie. Vol. 2. 1881

" Beitrage zur Kenntniss der Metamorphose

des Ammoccetcs branchialis in Petromyzon1'

Schneider's Beitrage. Vol. 2. 1890

" Entwickelungsgeschichte von Peripatus Ed-
icardsii und Peripat us torquatus" II. Theil

Arbeit, a. d. Zool. Zoot. Instit. WUrzburg.
Vol. 8. 1888

" On some Points in the Early Development
of Motor Nerve-trunks and Myotomes in

Lepidosiren paradoxa "

Trans. Roy. Soc. Edin. Vol. 41. 1904

" Zur Metamerie des Selachierkopfes "

Verhandl. d. Anat. Gescll. Yersamml. in
Milnchen, 1891

"On the Development of Linmlus longispina "
Journ. of Coll. of Sci., Tokio. Vol. 5. 1891

Quoted by Beard

" Die obere Trigeminus-Wurzel " . . . .
Arch. f. Mikr. Anat. Vol. 53. 1899

Handbuch der Gewebe-Lehre. 6th Auflage.
1893

" Rudimentare Wirbelthieraugen "
Bibliotheca Zoologica. Leukart und Chun.

Vol. 4 and Vol. 5
" Ueber den Bau und die Entwicklung der

sogenannten Carotis-driise "

Archiv . f. Mikr. Anat. Vol.56. 1900

Pages of
reference.

124, 258, 259
222, 225, 271

238
327

329, 339, 340,
351

345

444

425

135, 304
398, 399, 411

461, 466, 478
262

167, 238, 252,

253, 273, 320,

382

318

280

264, 425, 488
94,96,99,101

428

5o8

THE ORIGIN OF VERTEBRATES

Author's name.

KORSCHELT and
HEIDER . . .

KOWALEWSKY

KRIEGER .
V. KUPFFER

LANG . . .
LANGERHANS

LANGLEY .
LANKESTER

Title of Paper.

1889
Entwicklungs-

Text-book of the Embryology of the Inverte-
brates.' Translated by M. Bernard. 1900.
Part III. and Part IV

" Ein Beitrag zur Kenntniss der Excretions-

organe der Pantopoden "

Mem. d. I'Acad. d. Imp. d. Sci. d. St. Peters-
burg. Ser. VII. Vol. 38. 1890
" Une nouvelle glande lyrnphatique chez le
scorpion d'Europe "

Ibid. Ser. VIII. Vol. 5. 1897
" Etude Biologique sur les Clepsines "

Ibid. Ser. VIII. Vol. 5. 1897
" Ein Beitrag zur Kenntniss der Excretions-

organe "

Biologisches Centralblatt
" Weitere Studien iiber die
geschichte des Amphioxus lanceolatus"

Archiv. f. Mikr. Anat. Vol. 13. 1877
" Ueber das Centralnervensystern des Fluss-

kl°Gl)SGS . • • • •

Zeiisch. f. 'wis's. Zool. Vol. 33. 1880
' Studien zur vergleichenden Entwicklungs-
geschichte des Kopfes der Kranioten.'
Heft. 1. ' Die Entwicklung des Kopfes

von Acipenser '

Munchen. 1893
Heft. 2. ' Die Entwicklung des Kopfes
von Ammoccetes Planeri .

Munchen. 1894
Heft. 3. ' Die Entwicklung der Kopfner-
ven von Ammoccetes Planeri.'
Dritter Abschnitt. ' Die Metamor-
phose des larvalen Nervensystems

des Kopfes '

Munchen. 1895

Pages of
reference.

27, 73, 88,
114-116, 397,
429, 431

421

423
421

420, 422, 472

409, 410

101

318, 319, 320,
440

300, 440

' Text-book of Comparative Anatomy.' Trans-
lated by H. M. and M. Bernard ....
" Untersuchungen iiber Petromyzon Planeri"
Bericht v. d. Yerhandl. d. Naturforsch. Ge-

scllsch. z. Freiburg. 1873
Schafer's' Text-book of Physiology.' Vol.2. 1900
Article " Vertebrata " in the ' Encyclopaedia

Britannica '

" On the Skeleto-trophic Tissues and Coxal

Glands of Limulus, Scorpio, and Mygale .

Q. J. Micr. Sci. Vol. 24. 1884

" Limulus an Arachnid "

Q. J. Micr. Sci. Vol. 21. 1881

' Extinct Animals '

London. Constable & Co. 1906
A treatise on Zoology. Edited by E. Ray
Lankester.

Part II. ' The Entero-ccela and the
Ccelomoccela '

228, 263, 282,
283, 405, 458

357

94-101, 301,

405

2, 3, 448

484

137, 139, 253,
320, 321
62, 238, 241,
306, 361, 366
22, 150, 345

472-478

BIBLIOGRAPHY AND INDEX OF AUTHORS

509

Author's name.

LANKESTER and
POWRIE . . .

LANKESTER,
BEN HAM, and
BECK ....

LANKESTER and
BOURNE . . .

LANKESTER. and
WILLEY . . .

LANKESTER and
GULLAND . .

Title of Paper.

LATREILLE
LAURIE . .

" A Monograph of the Fishes of the Old Red
Sandstone of Britain."

Parti. " The Cephalaspidse " . . . .
Pakeontographical Soc. 1808

" On the Muscular and Endo-skeletal Systems
of Limulus and Scorpio, with some Notes
on the Anatomy and Generic Characters of

Scorpions "

Trans. Zool. Soc. Vol. 11. 1885

" The Minute Structure of the Lateral and
Central Eyes of Scorpio and Limulus " .

Q. J. Micr. Sci. Vol. 23

"The Development of the Atrial Chamber of

Amphioxus "

Q. J. Micr. Sci. Vol. 31. 1890

" Evidence in Favour of the View that the
Coxal Gland of Limulus and of other
Arachnids is a Modified Nephridium " .

Q. J. Micr. Sci. Vol. 25. 1885

Pages of

reference.

LEYDIG
LOCY .

" The Anatomy and Relations of the Eurypte-

ridse "

Trans. Roy. Soc. Edin. Vol. 37. 1893

" On a Silurian Scorpion and some Additional

Eurypterid Remains from the Pentland

Hills"

Ibid. Vol. 34. 1899

LOEB, LEO, and R.
M. STRONG . .

LOWNE

LUGARO
LWOFF

MAAS . . .
MACBRIDE .
McDOUGALL

" Contributions to the Structure and Develop-
ment of the Vertebrate Head " .

Journ. Morph. Vol.11. 1895

" On Regeneration in the Pigmented Skin of
the Frog, and on the Character of the

Chromatophores "

Artier. Jour, of Anat. Vol.3. 1904
' The Anatomy, Physiology, Morphology, and
Development of the Blow-fly '

London. 1895
Quoted by Anderson ........

" Ueber den Zusammenhang von Markrohr
und Chorda beim Amphioxus und ahnliche

Verhaltnisse bei Anneliden "

Zeitsch. f. u-iss. Zool. Vol. 56. 1893

" Ueber Entwicklungstadien der Vorniere und

Urniere bei Myxine "

Zool. Jahrbuch. Vol. 10. 1897
"Further Remarks 011 the Development of

Amphioxus"

Q. J. Micr. Sci. Vol. 43. 1900
See Hardy and McDougall.

29, 275, 327,
339, 345

177,222,224,
313

74, 81-83
409

429
221
237

238, 239
91
179, 262

470

369, 370, 375
467

444

392, 402, 412,
419

410

5io

THE ORIGIN OF VERTEBRATES

Author's name.

MACLEOD

MAGNUS . .

Title of Paper.

MARK . . .
MARSHALL

MASTERMAN
MAURER .

MAYER, F. .
MAYER, P. .

METSCHNIKOW .
MEYER ....

MILNE-EDWARDS

MINCHIN . . .
MITSUKURI . .

MOTT . . . .

MOTT and HALLI-
BURTON . . .

MULLER, J.

MULLER, W. .

NEAL

NESTLER

" Recherches sur la structure et la significa-
tion de l'appareil respiratoire des Arach-

nides "

Archiv. de Biol. Vol.5. 1881

" Versucho am Uberlebenden Diinudarrn von
Siiugetliieren "

Archiv. f. d. Ges. Pln/siologie. Vols. 102, 103.
1904

"On the Head-cavities and Associated Nerves

of Elasrnobranchs "

Q. J. Micr. Sci. Vol. 21. 1881

" The Segmental Value of the Cranial Nerves "
Journ. of Anat. and Physiol. Vol. 16. 1882

" On the Diplochorda "

Q. J. Micr. Sci, Vol. 43. 1900

" Die Schilddriise, Thymus und andere

Schlundspaltenderivate bei den Eidechse ".

Morph. Jahrbuch. Vol. 27. 1899

" Das Centralnervensystem von Ammoccetes "
Anat. Anzeig. Vol. 13. 1897

" Ueber die Entwicklung des Herzens und der
grossen Gefassstamme bei den Selachiern".

Mitth.a.d, Zool. Stat. z. Neapel. Vol.7. 1887

Quoted by Kowalewsky

" Studien iiber den Korperbau der Anneliden "

Mitth. a. d. Zool. Stat, z. Neapel. Vol. 7. 1887

" Anatomie des Limules "

Annales des Sciences Naturelles. Ser. 5. Vol.
17. 1872

A treatise on Zoology. Edited by Ray Lan-
kester. Part II. " The Porifera and Ccelen-
terata"

" On the Fate of the Blastopore, the Relations
of the Primitive Streak, and the Formation
of the Posterior End of the Embryo in

Chelonia," etc

Journ. Coll. Sci. Tokyo. Vol. 10. 1896

" Croonian Lectures of the Roy. Coll. of
Physicians," 1900

" On the Chemistry of Nerve-degeneration " .
Phil. Trans. Boy. Soc. B. Vol. 194. 1901

" Vergleichende Anatomie der Myxinoiden " .
Abhandl. d. Ecjl. Akad. d. Wiss. Berlin. 1834
" Ueber die Stammes Entwickelung des Sehor-

gans der Wirbelthiere "

Festgabe C. Ludwig. Leipzig. 1874

" The Segmentation of the Nervous System

in Squalus acanthias"

Bull, of Mus. Comp. Zool. Harvard. Vol. 31.

1898
" Beitrage zur Anatomie und Entwicklungs-

geschichte von Petromyzon Planeri "
Archiv. f. Naturgesch. Juhrgang, 56. Vol.1.

1890

Pages of
reference.

169, 174

447

115

185, 186
260
16

427, 428
489

179

422
403

157, 159, 176,
177, 313

473

179

469
469

126

96-100, 105,
108

179, 266, 300

168, 171, 175,
445

BIBLIOGRAPHY AND INDEX OF AUTHORS

511

Author's name.

NIESKOWSKI

NUSBAUM, J.

OBERSTEINER

OKEN . . .
OWEN . . .

PANDER

PARKER, G. H.

PARKER, W. K.
PATTEN . .

Title of Paper.

" Der Eurypterus Remipes aus den obersilu-
rischen Schichten der Insel Oesel " .

Arch. f. d. Naturkunde Liv-Ehst-und Kur-
lands. 1st Ser. Vol. 3. 1858

" Einige neue Thatsachen zur Entwicklungs-
geschichte des Hypophysis Cerebri bei

Saugethieren "

Anat. Anzeiger. Vol. 12. 1896

' Central Nervous System.' Translated by Hill.
1896

" Essays on tbe Conario-Hypopbysial Tract,
and tbe Aspects of tbe Body in Vertebrate
and Invertebrate Animals "

" On tbe Anatomy of tbe American King-crab

(Limulus polyphemus) "

Trans. Linn. Soc. Vol. 28. 1873

' Monograpbie der fossilen Fiscbe des Siluri-
scben Systems des russiscb-baltiscben Gou-

vernements '

St. Petersbourg. 1856
" Tbe Retina and Optic Ganglia in Decapods,

especially in Astacus "

Mitth. a. d. Zool. Stat. z. Neapel. Vol. 12. 1895
" Tbe Compound Eyes in Crustaceans " .
Bull, of Harvard Mus. of Comp. Zool. Vol.

20. 1890
" Tbe Function of tbe Lateral-line Organs in

Fishes"

Bull, of the Fisheries Bureau. Washington.

Vol. 24. 1904
" Studies on the Eyes of Arthropods " .
Journ. of Morphology. Vols. 1 and 2. 1887

and 1889
" On the Skeleton of the Marsipobranch

Fishes"

Phil. Trans. Roy. Soc. 1883

"On the Origin of Vertebrates from Arachnids "

Q. J. Micr. Sci. Vol. 31. 1890

" On the Morphology and Physiology of the

Brain and Sense-organs of Limulus "

Q. J. Micr. Sci. Vol. 35. 1893
" New Facts concerning Botbriolepis "

Biological Bulletin. Vol. 7. 1904
" On the Structure and Classification of tbe

Tremataspidae "

Mem. d. VAcad. Imp. d. Sci. de St. Petersbourg.

Vol. 13. 1903
" On the Structure of the Pteraspidse and

Cephalaspidse "

The American Naturalist. Vol. 37. 1903
" On the Appendages of Tremataspis "

The American Naturalist. Vol. 37. 1903
" On Structures Resembling Dermal Bones in

Limulus "

Anat. Anzeig. Vol. 9. 1894

Pages of
reference.

26, 239, 240

320

264, 280

258

211

327

91, 93, 97
99, 100, 114

357

73, 79, 83-85,
114

120, 125, 126,

131

352, 353

358-367, 371
32, 351, 450

329

415
351

346

5i2

THE ORIGIN OF VERTEBRATES

Author's name.

PATTEN AND
HAZEN .

PATTEN and RE-
DENBAUGH . .

PERLIA
PICK .
PLATT

PRICE

Title of Paper.

Pages of
reference.

RABL

RAMON Y. CAJAL
RATHKE . . .

REDENBAUGH
REICHENBACH

RETZIUS
ROHON .

ROLPH . .
RU'CKERT, J.

"The Development of the Coxal Gland, etc.,

of Limulus Polyphemus "

Journ. of Morphol. Vol. 16. 1900

Studies on Limulus. II. "The Nervous

System of Limitlns Poh/pliemns "...

Journ. of Morphol. Vol. 1G. 1900

Quoted by Edinger

ST. HILAIRE

"A Contribution to the Morphology of the
Vertebrate Head, based on a Study of

Acanthias vulgaris"

Journ. Morphol. Vol. 5. 1891

"Fibres connecting the Central Nervous
System and Chorda in Amphioxus "

Anat. Anzeig. 1892
" Development of the Excretory Organs of

Bdellostoma Stouti"

Zool. Jahrbuch. Vol. 10. 1897

" Ueber die Metamerie des Wirbelthierkopfes "
Verhandl. der Anat Gesellsch. Versamml. in

Wien. Anat. Anzeig. 1892
"Die Entwicklung und Structur der Neben-

nieren bei den Vogeln "

Arch, f.mikr. Anat. Vol.38. 1891

• ••••*•••••*••

" Anatomie des Querders "

Naturforsch. Gesellsch. zu Dantzig. Vol. 2.

1827
See Patten and Redenbaugh.
" Entwicklungs-geschichte des Flusskrebses "
Abhandl. d. Senckcnbergischen Naturforsch.

Gesellsch, Vol. 14. 1886.
'Biologische Untersuchungen.' Vol.1. 1890.

" Zur Kenntniss des Nervensystem der

Crustaceen "

Die Obersilurischen Fische von Oesel. 1st

Theil. " Thyestidse und Tremataspidse "
Mem. d. VAcad. Imp. d. Sci. d. St. Peter sbourg.

7th Ser. Vol. 38. 1892
" Weitere Mittheilungen iiber die Gattung

Thycstes"

Bull. d. VAcad. d. St. Petersbourg. 5th Ser.

Vol. 4. 1896
" Untersuchungen iiber den Bau des Amphi-
oxus lanceolatus "

Morphol. Jahrbuch. Vol. 2. 1887
" Entwicklung der Excretionsorgane "
Merkel und Bonnet; Anat, Hefte. Vol.1. 1891.
" Ueber die Entstebung der Excretionsorgane

bei Selachiern "

Archiv. f. Anatomie. 1888

" Sur la Vertebre "

La Revue EncyclopCdicpuc. 1822

403

314,315,381,

382

264

265

253, 265-267,
273, 274, 279,
284

443

394
258, 262

424

72, 465
161, 169, 304

98-100, 114

20, 489
32, 275, 276

327-330,339-
341, 382

444

392, 393, 400

403

BIBLIOGRAPHY AND INDEX OF AUTHORS

513

Author's name.

SAMASSA . . .
SCHAFFER . . .

SCHIMKEWITSCH

SCHMIDT . . .
SCHMIEDEBERG.
SCHNEIDER, A. .

SCHNEIDER, G. .

SCOTT . . . .
SEDGWICK . . .

SEMON .
SEMPER

Title of Paper.

SHELDON

" Bemerkungen iiber die Methode der Verglei-

chenden Entwicklungsgeschichte " .

Biol. Centralblatt. Vol. 18. 1898
" Ueber das Knorpelige Skelett von Arnmo-

coetes"

Zeitsch. f. iviss. Zool. Vol. 61. 1896
" Ueber die Thyrnusanlage bei Petromyzon

Plancri "

Sitzungsber. d. K. Akad d. Wiss. in Wien.

Vol. 103. 1894
" Sur la structure et sur la signification de

l'Endosternite des Aracbnides " .

Zool. Anzeig. 1893

" Anatoniie de l'Epeire "

Ann. d. Sci. Nat. Vol. 17. 18S4
" Die Crustaceen - fauna der Eurypteren -

scbichten von Rootzikiill auf Oesel "
Mem. d'Acad. Imp. d. Sci. d. St. Petersbourg.

Vol. 31. 1883
" Ueber die cbemiscbe Zusammensetzung des

Knorpels "

Arch. f. exper. Pathol, unci PJiarmak. Vol 28.

1891
' Beitrage zur Anatoniie und Entwicklungs-

gescbicbte der Wirbeltbiere '

Berlin. 1879

" Ueber phagocytare Organe und Chloragogen-

zellen der Oligochseta "

Zeitsch. f. wiss. Zool. Vol.61. 1896

"Notes on the Development of Petromyzon"
Journ. of Morphol. Vol. 1. 1887

" A Monograph of the Development of Peri-
pat us capensis "

Studies from the Morphological Laboratory,
Cambridge. Vol. 4. 1888

" Development of the Kidney in its Relation

to the Wolffian Body in the Chick " . .

Q. J. Micr. Sci. Vol. 20. 1880

" Early Development of the Wolffian Duct
and Anterior Wolffian Tubules in the Chick ;
with some Remarks on the Vertebrate Ex-
cretory System "

Q. J. Micr. Sci. Vol. 21. 1881

" Das Excretionssystern der Myxinoiden "

Festschrift f. Gegenbaur. Leipzig. 1897

" Die Stammesverwandschaft der Wirbel-
thiere und Wirbellosen "

Arbeit, a. d. Zool. Zoot. Inst. Wilrzburg.
Vol. 2. 1875

" Das Urinogenitalsystern der Plagiostomen
und seine Bedeutung fur die iibrigen Wirbel-
tbiere "

Ibid. Vol. 2. 1875

"On the Development of I'cripatus Nova-
Zealandia "

Studies from the Morphological Laboratory,
Cambridge. Vol. 4. 1889

Pages of
reference.

462

126-135

426-428

143-145, 342
369

190,191,236,
240, 329, 341

147

128, 130, 172,
195,197,213,
310, 445

421

42, 78, 111,
112, 406

397-400

390

393, 394, 400
400, 419

390, 392

400
2 L

THE ORIGIN OF VERTEBRATES

Author's name.

Title of Taper.

Pages of
reference.

SHERRINGTON .

" On the Anatomical Constitution of the

267

Joum. of Physiol. Vol. 17. 1894. Proc of

Physiol. Soc. June 23

SHIPLEY . . .

334

" On some points in the Development of

167,

305, 378,

Q. J. Micr. Sci. Vol. 27. 1887

401,

405, 406

v. SMIRNOW . .

" Ueber die Nervenendigungen in den Nieren

477

Anat. Anzeigcr. Vol. 19. 1901

SMITH, ELLIOT

SPANGENBERG .

" Zur Kenntniss von Branchipus stagnalis " .

Zcitsch. f. iciss. Zool. Vol. 25. 1875

396

SPENGEL . . .

Berlin. 1893

494

STARR ....

265,

266

STUDNICKA . .

" Sur les organes parietaux de Petromyzon

80, 81.

Sitzungsber. d. K. Gesell. d. Wiss. i)i Prag.

1893

" Ueber den feineren Bau der Parietalorgane

von Petromyzon marinus "

81,86

Sitzungsber. d. K. bohmischen Gesell. d. Wiss.

Prag. 1899

TAKAMINE . . .

" The Isolation of the Active Principle of the

423

Joum. of Physiol. Vol.27. Proc. of Physiol.

Soc, Dec. 14, 1901

TARNANI . . .

" On the Anatomy of the Thelyphonides "

Bcvue des Sciences Naturclles, St. Peters-
bourg. 1890

190,

206-208

" Die genitalen Organe der Thelyphonus " .

190,

206-208

Biol. Centralblatt. Vol. 9. 1889

TRAQUAIR . . .

" Report on Fossil Fishes collected by the
Geological Survey of Scotland in the Silu-

rian Rocks of the South of Scotland " .

343-

345, 350

Trans. Boy. Soc, Edin. Vol. 39. 1899

VIALLANES . .

" Contribution a l'histologie du systeme ner-
veux des Invertebres ; la lame ganglion-

100

Ann. Sci. Nat. Vol. 13

VINCENT, SWALE

" The Carotid Gland of Mammalia and its
Relation to the Supra-renal Capsule, with
some Remarks upon Internal Secretion and

the Phyogeny of the latter Organ "...

424

Anat. Anzeigcr. Vol. 18. 1900

"Contributions to the Comparative Anatomy

and Histology of the Supra-renal Capsules "

424

Trans. Zool. Soc. Vol. 14. 1897

VIRCHOW . . .

" Transformation and Descent "

Joum. of Path, and Bacter. Vol.1. 1893

479

VOGT .

258

VOLCKER . . .

See Hensen and Volcker.

WAGNER . . .

369

BIBLIOGRAPHY AND INDEX OF AUTHORS

515

Author's uauie.

Title of Paper.

Pages of
reference.

WEISS ....

"Excretory Tubules in Ampliioxus Lanceola-

te* "

420

Q. J. Micr. Sci. Vol. 81. 1890

WELDON . . .

" On the Supra- renal Bodies of Vertebrates "
Q. J. Micr. Sci: Vol. 25. 1885
" Note on the Origin of the Supra-renal

420, 424, 429

424

Proc. Roy. Soc. Vol. 37. 1884

WHEELER . . .

" Development of the Urino-genital Organs

402, 405

Zool. Jahrbuch. Vol. 13. 1899

v. WTJHE . . .

" Ueber die Mesoderrnsegmente des Rumpfes
und die Entwicklung des Excretionsystems

bei Selachiern "

155-157, 172,

Archiv. f. Mikr. Anat. Vol. 33. 1889

173, 188, 234,
258, 260, 262,
263, 266, 273,
280,308,390-

393, 397, 400,
406-408, 412

" Beitrage zur Anatomie der Kopf region des

410, 426-428

Petrus Camper. Deel. 1 ; Aflevering. 2

WILLEY . . .

See Lankester and Willey.

WOLFF ....

" Die Cuticula der Wirbelthierepidermis "
Jen. Zeitsch. f. Natiirwissenschaft. Vol. 23.
1889

302

WOODWARD, H. .

" A Monograph of the British Fossil Crus-

tacea, belonging to the order Merostornata "

235-240, 249

Palceontographical Society. 1878

251, 275

WOODWARD,

SMITH

' Catalogue of Fossil Fishes in the British

339

29, 326, 327,

London. 1891

344, 349, 351

v. ZITTEL . . .

Handbuch der Paheontologie

190

GENERAL INDEX

[The numbers in dark type refer to illustrations]

Acilius larva, eye of, 78, 83
Acromegaly, 425
Actinotrocha, 438
Addison's disease, 423
Adelopthalmus, 249
Adrenalin, 423, 491
Adrenals, 423, 491
Agnathostornatous fishes, 29, 343
Alimentary canal, 433

„ ,, Ammoccetes, 168, 405, 445

,, ,, invertebrate, compared to tube of central nervous system of verte-

brate, 43, 433
,, ,, innervation of, 447

origin of, 444
,, ,, position of vertebrate and invertebrate, 10

,, ,, possibility of formation of new, 58

,, ,, relationship between notochord and, 434

Ammoccetes, 32, 245

,, an ancestral type, 35, 309

,, alimentary canal, 168, 405, 445

,, auditory organ, 378, 379

brain, 39, 40, 41, 45, 46, 48, 54, 61
„ branchial appendages, 161, 162, 163, 164

basket-work, 126, 128, 296, 331, 335
chamber, 161, 168, 162, 163
,, ,, circulation in Limulus and, 174

,, ,, diaphragms, 161, 167

„ ,, lamellae, 175

,, ,, muscles, 171

,, ,, nerves, 164

segments, 178, 312
cartilage, hard, 133, 133, 293, 294, 377

muco, 130, 131, 291, 293, 294, 296, 330, 331, 333, 334,

335, 338
soft, 129, 130, 293, 294, 296, 335
,, degeneracy, evidence of, 59, 94, 343

,, development, 228, 458

,, digestion, 58, 442

,, epithelial cells of gills, 214

5 18 THE ORIGIN OF VERTEBRATES

Ammoccetes, epithelial cells of skin, 347

pits, 173, 200
eye, 93
,, „ muscles, 267

,, median or pineal, 63, 75, 76, 77, 78, SO, 85, 86
„ left, 78, 79
fat-column, 181, 182
,, „ in degenerated muco-cartilage, 333, 334

„ ganglia in embryo, 229, 283

gland-tissue round the brain, 209, 210, 379
head-region, 128, 162, 163, 193, 293, 294, 296, 298, 335
head-shield, 329, 331, 338
liver, 442, 452

lymphatic glandular tissue, 426
„ Mvillerian fibres, 489

,, muscles, eye, 173, 267

,, ,, lip, lower, 297

„ „ upper, 305

,, respiratory, 171

somatic, 332, 336, 409
tubular, 173, 298, 309
,, nerves, cranial, 141

facial, 186, 311
,. „ glossopharyngeal, 186

,, ,, optic, 105

trigeminal, 232, 288, 288
vagus, 153, 173, 186
nerve-fibres, medullation of, 20
notochord, 182, 435
olfactory tube, 219, 225, 227, 317
oral chamber, 317, 243, 287, 458
parasitism, 60, 286
pituitary, 321
,, prosomatic region, 243

,, pronephric duct, 402, 405

relationship to Ostracodermata, 326, 338, 344, 414, 416
,, retina, 93, 111

skin, 58, 346, 348, 442
skeleton, 125, 126, 132, 291, 296, 335
,, segments, comparison with segments of Eurypterus, 323

„ „ facial, 201

hyoid, 186, 201
„ „ prosomatic, 286

,, septa between myomeres, 416

„ tentacles of upper lip, 303

,, test, biological, to show relationship with Limulus, 493

thyroid, 192, 194, 196, 205, 213, 430
transformation, 18, 59, 125, 168, 193, 199, 200, 220, 227, 228, 287, 291,

304, 307, 309, 331, 336, 347, 349, 389, 445
velum, 228, 289, 298, 302
Amcebocytes, 473
Amphibia, 23, 345

GENERAL INDEX 519

Amphioxus, 33, 407

„ atrial cavity, 409

,, branchial nephric glands, 426

endostyle, 198, 212
,, excretory organs, 389, 395, 477

„ neuropore, 220, 457

notochord, 435, 436, 443
„ pleural folds, 495

,, septa between myomeres, 416

,, somatic muscles, 409

yolk, 485
Androctonus, 53, 54, 372, 423
Annelids, lateral sense-organs, 357, 367
,, nephric organs, 390
„ origin of Arthropods from, 395
,, parapodal ganglia, 283
,, phagocytic glands, 421
Anthozoa, 474
Antiarcha, 29, 326, 343
Antibody, 492
Antitoxin, 492
Anus, 43, 457
Aponeuroses, 327, 342, 414
Apparatus, auditory, 355
,, dioptric, 83

,, respiratory, 148

,, suctorial, of Petromyzon, 287
Appendages, branchial, of Ammoccetes, 161, 162, 163, 164
,, ,, Limulus, 164

,, ,, internal, 149

,, derivation of suctorial apparatus of Petromyzon from, 290

disappearance of, in transformation of Arthropod into Vertebrate, 386,
413
„ evidence of, in prosomatic region of ancient fishes, 342

,, muscles, in Limulus and Scorpion, 247

,, prosomatic, of Gigantostraca, 234

Trilobites, 351
Apus, 28, 137, 436, 437
Arachnids, eyes, 75, 87

,, diverticula of stomach, 109

,, lyriform organs, 364, 368

,, segmental excretory organs, 423

Archreocytes, 473
Artemia, 0. Branchipus
Arthropleura, 249

Arthropoda, arrangement of organs, 10
,, evolution, 11

., excretory organs, 396, 418

eyes, 75, 89
giant-fibres, 489
,, musculature, 411

,, olfactory organs, 220

520 THE ORIGIN OF VERTEBRATES

Arthropoda, resemblance to ancient fishes, 29
Astacus, brain, 54

„ digestive ferment in cells lining the carapace, 442

,, optic chiasma, 101

,, optic stalk, 91

,, retina, 98
Asterolepis, 326, 342
Atrium, 410

Auchenaspis (Thyestes), 30, 31, 75, 275, 326, 327, 328, 338
Auditory apparatus, 355
Auerbach, plexus of, 447
Aurelia, 475
Autonomic nerves, 3

Balanoglossus, 12, 12, 433, 438, 494
Bdellostoma, 394, 405
Belinurus, 24, 249, 351
Bird, rhomboidal sinus, 46
Bladder, 449

„ swim, 148
Blastula, 459, 471, 473
Blood, 463, 472, 474

„ circulation, in Ammoccetes and Limulus, 174
,, secretion of ductless glands into, 418
Bothriolepis, 29, 32, 239, 326, 351, 450
Bone, 344, 474, 481
Brain, Ammoccetes and Arthropod, 54, 61

,, and brain-case of Ammoccetes, 40, 41, 46, 209
,, caudal, of Thelyphonus, 450
,, epithelial lining of, 38
,, roof, 39

,, Sphseroma serratum, 62, 90
,, Thelyphonus, 56
., ventricles, 4
,, vesicles, 48
Branchial basket-work of Ammoccetes, 126, 128, 296, 331, 335
Branchipus, 28

,, brain, 51, 54

,, eyes, lateral, 88

„ ,, retina of, 91, 97

,, ,, median, 75

,, excretory organs, 396

„ (Artemia) diverticula of gut and retinal ganglion, 110, 111, 113

,, nerves of appendages, 157

,, segmentation, 159

,, resemblance to Trilobite, 436

Bunodes, 24, 30, 249, 341, 351, 414
Bundle of Meynert, 48, 77
Bundles, posterior longitudinal, 489
Buthus, muscles, 270

Calcification in aponeuroses of Cephalaspis, 414

GENERAL INDEX 52 I

Calcification in cartilage, 140, 330

,, successive layers of the skin, 348

Camerostome, 221, 222, 223, 224, 241, 271
Canal, alimentary, formation of vertebrate, 58, 433, 446
,, ,, innervation, 447

„ ,, relationships between notochord and, 434

origin, 444
„ Haversian, 329

,, central, of spinal cord, 405, 439, 455
,, spinal, 182
Capsule, auditory, 377, 379

Cartilage Ammocoetes, rnuco, 127, 130, 131, 200, 291, 303, 330, 333, 334, 344

hard, 133, 133, 377
soft, 126, 129, 130
,, ,, spinal cartilages, 414

,, Hypoctonus, 133, 142
,, Limulus, hard, 142
,, ,, rnuco, 139

soft, 20, 130, 137
,, origin, 474, 481

,, staining reactions, 131, 133, 139, 330, 336
Cavity, atrial, 409, 413

„ coelomic, 167, 251, 266, 320, 339, 391, 408, 422, 430, 472
Cells, free-living, 463
Centre, vaso-motor, 468
Cephalaspis, diverticula of gut, 109
,, eyes, lateral, 75, 275

,, „ median, 75

head-shield, 327, 328, 330, 338
,, muscles on head-shield, 269

resemblance to Ammocoetes, 145, 291, 326, 329, 338, 348, 414
„ ,, Arthropod, 29

,, segmentation, 339

Ceratodus, 148
Cephalization, 51
Cephalodiscus, 438
Cephalopod, 23
Cerebellum, 47, 50
Chaetopoda, 395

Chamber, oral, of Ammocoetes, 243, 287, 458
Cheliceree, 235
Chiasma, optic, 101
Chilaria, 235, 238, 291, 301, 458

Chitin, 85, 119, 139, 205, 206, 302, 329, 346, 359, 440, 443
Cilia, 206

Circulation, branchial, 174
Cirri, 357

Clarke's column, 467
Clepsine, nephridial glands, 423
Cochlea, 378
Coelenterata, 465, 472
Ccelolepidas, 344

52 2 THE ORIGIN OF VERTEBRATES

Ccelom, 167, 251, 400, 472, 481
Ccelomata, 472
Coelomoccela, 472, 475
Coeloinostomes, 477, 481
Colleneytes, 474
Commissure, anterior, 49

„ oesophageal, 14

„ posterior, 48, 280

Comparison of brains of Ammocoetes and Arthropod, Gl

,, ,, invertebrate from Branchipus to Ammocoetes, 54

„ ,, vertebrate, 40

„ branchial circulation in Ammocoetes and Limulus, 174

„ ,, lamellse of Scorpion and Ammocoetes, 175

,, ,, segments of Ammocoetes and Petromyzon, 169

,, Cephalaspidian and Palaeostracan fish, 31

,, Ccelom of Peripatus and Vertebrate, 400

., dermal covering of Pteraspis with chitin of Limulus or dentine of fish

scales, 346
,, entosternite or plastron of Limulus with trabeculse of Ammocoetes,

145
,, excretory organs of vertebrates and invertebrates, 389

,, gut of Arthropod and tube of central nervous system of Vertebrate, 43,

244, 433, 440, 455, 457
,, head-shield of Cephalaspis and Ammocoetes, 291, 329, 338

,, hypophysial tube with olfactory tube of Arthropod ancestor, 229

,, ,, ,, with position of pala;ostoma, 317

,, mesosomatic region of Ammocoetes and Eurypterus, 192

,, muscles, branchial, of Ammocoetes and appendage muscles of Scorpion,

171, 447
,, ,, eye, of Vertebrate with dorso-ventral muscles of Scorpion,

267, 272, 459
,, ,, of oral chamber of Ammocoetes and prosomatic musculature

of Limulus, 247, 447
,, ,, longitudinal body-muscles of Vertebrate and dorsal longi-

tudinal muscles of Arthropod, 411, 447
,, nerves, appendage of Limulus and Branchipus to lateral root system

of Vertebrate, 157
,, ,, cranial and spinal segmental, 152

,, nervous systems of Vertebrate and Arthropod, 36

,, pineal gland of vertebrates and median eyes of Arthropod, 63, 456

,, pituitary body and coxal glands, 246, 319, 321

,, prosoma and mesosoma of Limulus and Ammocoetes, 140, 141

,, prosomatic region of Ammocoetes and Eurypterus, 244, 333

,, retina in Ammocoetes and Musca, 97

,, ,, compound in Arthropod and Vertebrate, 87

,, skeleton of Limulus and Ammocoetes, 126, 136

,, sense-organs of Arthropod appendages with auditory organs of

Vertebrate, 375
,, thyroid with endostyle, 198

,, ,, ,, uterus of Scorpion, 205

Corneagen, 69
Corpora quadrigemina, 47

GENERAL INDEX S23

Corpuscles, Pacinian, Herbst, Grandry, etc., 470
Coxal glands, 242, 246, 319, 321, 389, 398, 403, 429
Cranium, 121, 145, 339
Crayfish, 442, 489
Crest, neural, 281
Cromatophores of frog, 470
Crura cerebri, 14
Crustacea, first appearance, 27
eyes, 76, 87

,, retina, 100

,, segmental glands, 422

Ctenophora, 474
Cyathaspis,'29, 326, 340, 343
Cyclostomata, 165, 229, 343, 353, 424
Cysts, 50

Daphnia, 112

Degeneration, 17, 19, 59, 74, 78, 94, 107, 212, 309, 333, 336, 343

Deiters' nucleus, 489

Dendrites, 72

Development, parallel, 497

„ of two types of eye, 73

„ vertebrate retina, 101

Diaphragms, 161, 167
Didymaspis, 327, 338
Digestion, 441
Dinosaurs, 17
Dipnoans, 23, 45, 148
Diptera, 89, 369
Diverticula, optic, 102
Dogfish, skull, 121, 123
Drepanaspis, 344, 345, 450
Drepanopterus Bembycoides, 238

Ectognath, 238, 242, 271, 304, 342, 381
Eel, 488

Elasmobranchs, 23, 343, 423
' Elastin, 435
Embryo, head of dogfish, 121, 123

skull of pig, 121
Embryology, principles of, 455
Encepalomeres, 262
Endognath, 238, 271, 304, 381
Endostoma, 241, 306
Endostyle, 198, 212
Entapophysis of Limulus, 139
Enteroccela, 472
Enteropneusta, 438, 494
Entochondrites, 377
Entosclerite, 222, 271
Entosternite, 143
Epiblast, 444, 445, 459

524 THE ORIGIN OF VERTEBRATES

Epithelium cells of Ainmoccetes, 347

,, of central nervous system of vertebrates, 38, 457

,, ccelomic spaces in annelids, 421

,, optic diverticula, 103

„ peritoneal, pleural, and pericardial cavities, 477

,, velum of Ammocoetes, 301, 302

Equilibration, 358
Eukeraspis, 326
Eurypterus, 26, 150, 191, 237

,, appendages, 150, 236, 237

,, classification, 249

,, comparison with Ammocoetes, 170, 323

,, diagram of sagittal median section, 240, 245

,, endostoma, 241, 306

„ eyes, 275

,, mesosomatic segments, 192

,, muscles of carapace, 269

operculum, 150, 190, 212
Evidence of alimentary canal, innervation, 446

„ auditory apparatus and lateral line organs, 355

,, ccelomic cavities in Limulus, 251

,, degeneracy in Ammocoetes, 59, 94, 343

,, embryology, cartilage, 20, 129

,, ,, eye-muscles, 263

,, ,, excretory organs, 390

heart, 179, 451

,, ,, nervous system, central, cerebral vesicles, 48, 458

epithelial tube, 37, 42, 102, 244, 433,
455

,, ,, ,, ,, ,, neurenteric canal, 37

„ ,, ,, ,, ,, neuropore, 220, 457

,, ,, ,, ,, ,, optic diverticula, 102

spinal cord, 46
oral chamber, 228, 242, 243, 290
olfactory organ, 220, 227

,, ,, palseostoma or old mouth, 317

,, ,, pineal or median eyes, 15, 63, 74, 456

,, ,, pituitary body and coxal glands, 246, 319

thyroid, 192, 194

,, ,, segmentation, double, of head, 157, 234, 258

„ ,, skeleton, cranial, 120, 153

,, nervous system, central, 8

,, notochord, origin from segmented region, 443

,, olfactory apparatus, 218

,, organs of vision, 68

„ palaeontology, 20, 497

,, pineal or median eyes, 74

,, prosomatic musculature, 247

,, respiratory apparatus, 148

,, segmentation in head-shield, 339

,, skeleton, 119

Evolution, 8, 15, 20, 149, 482, 497

GENERAL INDEX 525

Evolution of brain in brain-case, 210

cranium of Vertebrate, 342

excretory organs, 389

eye of Vertebrate, 114

nervous system, central, 34

tissues, 19

Vertebrate from Balanoglossus and Ampbioxus, 33
Eyes, 68

„ lateral, 87, 105, 108

,, median or pineal, 74, 77, 78, 79

Fat-cells in muco-cartilage, 332
Fat-column of Ammoccetes, 181, 182
Fibres, Mautbnerian, 488

,, Miillerian, of Ammoccetes central nervous system, 489
retina, 96, 107
Fisbes, classification, 218

,, ancient, classification, 326, 343
,, ,, cloacal region, 450

,, ,, dominance, 23

eyes, 75
,, ,, bead-sbields. See Head-sbields

,, ,, pleural folds, 414

Fissure, posterior, 43
Fittest, survival of, 16, 34
Flabellum, 359, 360, 362, 363, 366
Folds, pleural, 410, 414
Function of auditory organ, double, 358
,, lateral line sense-organs, 357

,, nerves, 448

tbyroid, 212, 215
Fusion of ganglia, 52

Galeodes, 230

„ brain, and camerostome, 222, 223

,, primordial cranium, 341

,, racquet-organs, 369, 375

Ganglia, infraoesopbageal, 4, 12, 14, 51, 221

supraoesopbageal, 4, 12, 14, 49, 52, 221, 225
,, origin of, of cranial and spinal nerves, 281
Ganglion, epibrancbial, 164, 282
,, babenulse, 48, 78

optic of retina, 72, 89, 97
,, of posterior root, 466

cells of sympathetic system, 424, 428, 448
Ganoids, 23, 345
Gastrula theory, 165, 459
Genital corpuscles, 470
Geological record, 20
,, strata, 22
Geotria australis, 80
Germ-band, 482

526 THE ORIGIN OF VERTEBRATES

Germ-cells, 471
Giant-fibres, 489
Gigantostraca, 25, 234
Gills, 148, 161, 185, 214, 494
Glabellurn, 339
Glands, carotid, 427

coxal, 242, 246, 319, 321, 425, 429

ductless, 418

generative, of Limulus, 209

internal secretion of, 214

lymphatic, 418

pineal, 15, 63, 75, 456

pituitary, 244, 246, 319, 425

segmental, of Crustacea, 422

submaxillary, 466

sweat, 448

thymus, 425

thyroid, of Aminoccetes, 193, 194, 196, 201, 205, 429

tissue round brain of Ammoccetes, 209, 379

uterine, of Scorpion, 202, 203, 204, 205
Gnathostomata, 60, 343
Goblet, 359, 360, 373
Goitre, 215
Gonad, 475, 479
Gonoccele, 475, 481
Grooves, ciliated, 188, 197, 212

,, hyper-pharyngeal of Amphioxus, 410
,, ventral, of apus and trilobites, 436
Gymnophiona, 393

Hjemocytes, 472
Head of embryo dogfish, 121, 123
Head-shield, dorsal, of Ammoccetes, 330, 331, 338
,, ,, Auchenaspis, 29, 31, 338

Cephalaspis, 327, 328, 330, 338, 348
,, „ Cyathaspis, 340

,, ,, Didyniaspis, 338

,, „ evidence of segmentation, 339

,, ,, Keraspis, 328

„ ,, Ostreostraci, 327, 348

,, ,, Palseostracan, 348

„ ,, Pteraspis, 29

Thyestes, 29, 31, 327, 332, 338, 340, 341, 348
,, ventral, Scaphaspis, 349

Heart, nerves, 2, 447

,, origin of vertebrate, 179, 451, 459
,, relative position in vertebrate and invertebrate, 175
„ veins forming vertebrate, 180
Hemiaspis, 24, 25, 249, 250, 351, 414
Hemispheres, cerebral, 47
Hepatopancreas of Ammoccetes, 452
,, Limulus, 211

GENERAL INDEX 527

Heterostraci, 29, 275, 326, 343

Hirudinea, 478

Histolysis in transformation of the lamprey, 59

Homology of branchial region of vertebrate and invertebrate, 149

,, ductless glands and nephridial organs, 418

,, external genital ducts of arthropods and nephridia of annelids, 429

,, germinal layers in all Metozoa, 459

,, pituitary body of Arnrnoccetes and coxal glands of Limulus, 319

,, tubular muscles of Arnrnoccetes and veno-pericardial muscles of

Limulus, 309

,, ventral aorta of vertebrate and longitudinal venous sinuses of Limulus,

178
Hydra, 441, 465, 472, 470
Hydrophilus larva, eye, 84
Hyoid segment in Arnrnoccetes, 186, 267
Hypoblast, 434, 438, 444, 445, 459
Hypoctonus, cartilage cells in entosternite, 133

operculum, 189, 207
Hypogastric plexus, 3
Hypogeophis, 393
Hypophysis, 229, 244, 317, 318, 340

Inpundibulum, position, 122, 132

,, tube, the ancestral oesophagus, 4, 37, 244, 318

,, ,, relation to neural canal, 14, 36, 318, 440, 457

notochord, 318, 435, 440
olfactory tube, 220, 22S, 318, 340
Insects, chordotonal organs, 364, 370
Invertebrate, heart, 175, 179

,, excretory organs, 418

,, nervous system, 13, 54

segmental nerves, 152

-,-,1

Kekaspis, 75, 328, 338
Kidney, 420, 459, 476

,, nerves, 477
King-crab, v. Limulus

Labykinthodont, 21, 28

Lamina terminalis, 49

Lamprey, v. Arnrnoccetes and Petromyzon

Larva, v. Transformation of the Lamprey

Lateral line system, 261, 355, 411, 470

Law of Progress, 19

,, Recapitulation, 434, 456, 498
Layer, germinal, 459

,, laminated, 347, 348
Leech, 421

Lens, formation, 83, 115
Lepidosiren, 148, 461, 466
Limulus or king-crab, 25, 140, 236, 240

„ appendages, branchial, 138, 164, 175

528 THE ORIGIN OF VERTEBRATES

Limulus appendages, prosomatic, 381

„ brain, 54

,, circulation, 174, 176

„ classification, 26, 249

,, ccelomic cavities, 252, 328

„ coxal glands, 321, 389, 397, 403, 429

,, eyes, median, 62, 74, 81

,, entosternite or plastron, 142, 143

„ flabellum, 360, 362, 363, 380, 381

,, generative organs and ducts, 189, 202, 208, 209, 380

,,. heart, 180

,, musculature, branchial, 170

,, „ prosomatic, 247

„ veno-pericardial, 177, 297, 309, 313

,, nerves, appendage, 140, 157

,, ,, cardiac, 314

,, ,, segmental, tripartite division of, 157, 235, 267, 355

,, segments, branchial, 152

,, ,, first mesosomatic, 188

,, ,, prosomatic, 233

„ operculum, 189, 202, 235, 295

,, sense-organs, poriferous, of appendages, 359
Lip, lower, of Ammoccetes, 246, 289, 297, 458
„ upper, „ 228, 243, 303, 336

Liver, Ammoccetes, 452
„ Limulus, 209, 211
Lizard, pineal eye, 80
,, suprarenals, 424
,, tail, 50
Lobes, optic, 101
Lobster, 489
Lungs, 148

Lung-books of scorpions, 150
Lymph, 474

Lymph-corpuscles, 463, 490
Lymphocytes, 472

Malapterueus, 470

Mammal, dominance of, 21

Man, dominance of, 17

Marsipobranchs, 23, 35

Medullation of nerve-fibres, 20, 267, 467, 477.

Membranes, basement, 436

Meroblastic egg, 485

Merostomata, 25, 249, 321

Mesencepalon, 48

Mesoblast, 444, 455, 459

Mesoglcea, 474

Mesonephros, 389, 400, 424, 429

Mesosoma, 52

Mesotheliurn, 472, 477

Metanephros, 389

GENERAL INDEX 529

Metasoma, 52, 387, 411

Metastoma, 239, 246, 272, 289, 342, 458

Metazoa, 444, 459, 471, 472

Meynert's bundle, 48, 77

Mollusca, dominance of, 23

Mouth, old, or paheostoina, 14, 317, 322, 440, 458

,, vertebrate, 317
Muco-cartilage, v. Cartilage
Muscles, antagonistic, 447

,, branchial, 170

,, connection of, with central nervous system, 464

,, eye, and their nerves, 263

,, prosomatic, 243, 247

,, phylogeny of origin of skeletal. 47S

,, rudimentary, in Amrnoccetes, 289

,, somatic trunk, origin of, 406

., striated, 20, 155

,, tubular, of Amrnoccetes, 309

„ unstriped, 20, 447, 491

,, visceral and parietal, 155, 172

,, veno-pericardial of Limulus and Scorpion, 177, 297, 309
Muscle-spindles, 267
Mygalidse, stomach, 109

„ segmentation, 249, 306
Myomeres, 262, 337, 414, 479
Myotomes, 332, 337, 338, 391, 407, 408
Mysis, eyes, 100

,, ductless glands, 422
Myxine, 220, 392, 402, 419

Nebalia, 144, 422
Nemertina, 475
Nephridia, 395, 421, 429
Nephrocoele, 430
Nephrotome, 393
Nerves, abducens, 155, 263, 266
auditory, 356, 376
,, autonomic, 3

facial, 155, 156, 186, 188, 192, 311, 356, 378
,, ,, ramus branchialis profundus, 311

,, to flabellum, in Limulus, 361, 375
,, glossopharyngeal, 155, 156, 186, 356
,, hypoglossal, 156
,, inhibitory, 447

inedullation of, 20, 267, 467, 477
occulomotor, 155, 234, 263, 274
,, olfactory, 229
optic, 101, 104
„ ,, of pineal eye, 79

,, origin of ganglia of cranial and spinal, 281
,, to pecten of Scorpion, 375, 376
,, preganglionic, 2

2 M

53° THE ORIGIN OF VERTEBRATES

Nerves, of prosoma in Limulus, 235, 355
,, regeneration of, 469
,, roots, of Limulus, 157
,, sacral, 448
,, segmental, 152, 156
,, segmental nature of cranial, 259, 411
,, spinal, absence of lateral roots in, 388
,, spinal accessory, 154

trigeminal, 151, 155, 156, 234, 243, 257, 279
„ motor nucleus of, 280

,, ,, of Ammoccetes, 288

tripartite arrangement of cranial nerves, 154, 157, 235, 267, 355
trochlear, 48, 155, 234, 263, 276
vagus, 151, 154, 156, 173, 186, 356, 447, 449
Nervous system, central, comparison of Vertebrate and Arthropod, 36, 457

,, ,, connection of, with muscular and epithelial tissues, 464

,, „ ,, with retina, 71

„ „ disease of, 50

„ „ evidence of, 8

„ ,, evolution of, 34

,, ,, importance of, 16, 463, 482, 498

,, ,, invertebrate, 10, 13, 54

,, ,, origin of, 480

„ ,, relation of germ-band to, 483

,, ,, segmentation of vertebrate, 51

tube of, 36-51, 102, 211, 433, 455, 457
vertebrate, 10, 13, 40, 41, 152
,, enteric, 447

sympathetic, 2, 424, 428, 448, 491
Neurenteric canal, 37
Neuroblast, 465

Neuromeres, 55, 247, 262, 312, 316
Neurones, 72, 92, 465
Neuropil, 71, 91
Neuropore, 220, 457
Nose, 219

„ of Osteostraci, 329, 352, 458
Notochord, 120, 122, 180, 181, 220, 244, 295, 318, 405, 417, 433, 436, 494

Ocelli, 70

(Esophagus of Ammoccetes, 405

,, Arthropod, compared to tube of infundibulum, 4, 244, 440

Olfactory apparatus, evidence of the, 218
,, organs of the Scorpion group, 220

tube of Ammoccetes, 219, 225, 244, 317
Oligochseta, 421, 478
Operculum of Eurypterus, 191, 212, 291
Limulus, 189, 202, 235, 295
,, Phrynus, 191

Scorpion, 189, 206, 212, 372
Thelyphonus, 189, 190, 206
Organs, arrangement of, 10

GENERAL INDEX

531

Organs, auditory, of arachnids and Insects, 368
branchial, innervation of vertebrate, 151

,, sense-organs of embryo vertebrate, 261, 281

chordotonal, of insects, 364, 369, 370
electric, 470
generative, of Limulus, 208, 209

,, connection between Thyroid gland and, 215

genital, of sea-scorpions, 206
lateral line, 355, 411
lyriform, of arachnids, 364, 369
olfactory, of Scorpion group, 220
phagocytic, 420
racquet, of Galeodes, 369, 375
segmental excretory, 389, 391, 408, 418, 459, 477
sense, of appendages of Limulus, 358
vestigial, 456
of vision, evidence of, 68
vital, 57
Origin of alimentary canal, 444

arthropods from annelids, 395

atrial cavity, 409

auditory capsules and parachordals, 377

coelom, 475, 481

ductless glands, 428

free cells, 472

heart of vertebrate, 179

lateral line organs, 356

muscles, 478

musculature, branchial, 170

,, somatic trunk, 406

nervous system, central, 480
notochord, 434

segmental excretory organs, 389
skeleton of vertebrates, 119
vertebrates, 9, 36, 351, 433, 493
Ostracodermata, 326, 343
Osteostraci, 29, 75, 275, 326, 343
Otoliths, 378
Ovum, 473

Pacinian bodies, 470, 477
Palannon, 20, 422
Palaeontology, evidence of, 20, 497
Palseostoma, 317
Palffiostraca, 27, 396

,, median eyes, 74

,, mesosomatic appendages, 188

,, olfactory organs, 221

,, segments, compared to Ammoccetes, 30S

Pantopoda, glands, 423
Parachordals, 121, 132, 377
Parapodia, 357

532 THE ORIGIN OF VERTEBRATES

Parapodopsis, foot glands, 422
Parathymus, 427
Parathyroids, 427
Parietal organ, 76

Pecten of scorpion, 114, 359, 366, 371, 372, 373, 374
Pedipalpi, 190
Periblast, 471

Peripatus, 396, 399, 400, 411, 421, 429
Petromyzon, alimentary canal, 405, 445
,, auditory organ, 378

,, branchial segments, 169

,, life-history, 59

olfactory tube, 219, 226
,, pronephric duct, 402

,, retina and optic nerve, 95

,, skeleton, 125

,, suctorial apparatus, 287, 304

,, transformation, v. Transformation of the Lamprey

Phagocytes, 420, 471
Pharynx of Amphioxus, 410
,, Vertebrate, 440

Phoronis, 439
Phrynus, brain, 53

,, caudal brain, 450
,, carapace and carapace removed, 250
,, ccecal diverticula, 109

,, evidence of segmentation of carapace, 249, 250, 341
,, operculum, 191
,, prosomatic appendages, 306
„ crossing of dorso-ventral muscles, 271, 277
,, stridulating apparatus, 368
Phyllodoce, 395
Phyllopoda, 321
Pigment, in Ammoccetes, in position of atrial cavity, 412

epithelial lining of central nervous system, 43, 457
choroid of vertebrate eye, 104, 107

between glandular cells round brain of Ammoccetes, 211, 379
tapetal layer of retina, 70
white, of right pineal eye of Lamprey, 76, 80
Pineal body, 14, 15
„ eyes, 74, 233, 244
,, ,, of Ammoccetes, 80, 78, 85
,, gland, 63, 75, 456
Pits, epithelial, of diaphragms in Ammoccetes, 164

,, ,, skin in Ammoccetes, 173, 200

Pituitary body, 244, 246, 319, 321, 425, 430
Plasma-cells, 471
Plakodes, 283
Planarians, 475

Plastron, formation of cranial walls from the, 86, 322, 341
of Limulus, 136, 142, 143
,, Palaeostracan, compared to trabecule of Ammoccetes, 145, 377

GENERAL INDEX 533

Plastron, muscles attached to the, 270

,, of Thelyphonus, 143
Platyhelmia, 475
Pleuron, 410, 415
Plexus, of Auerbach, 447

choroid, 38, 45, 49, 103

„ hypogastric, 3
Polychseta, 357, 395
Pores, abdominal, 430
Porifera, 473

Pouch, formation of gill, 165, 166
Prestwichia, 24, 25, 249, 351
Principle of concentration and cephalization, 51

,, embryology, 455

Pristiurus, 424
Progress, law of, 19

„ result of, 56
Pronephros, 389, 397, 419, 424, 449
Prosencephalon, 48
Prosoma, 52
Protopterus, 148
Protostraca, 27, 396, 417

,, dominance of, 28

Protozoa, 166, 479
Pseudoniscus, 25, 249
Pteraspis, 29, 30, 275, 326, 343, 344, 350
Pterichthys, 29, 31, 239, 326, 351
Pterygoid, pedicle of, 295

Pterygotus, 25, 27, 56, 170, 191, 221, 235, 238, 249, 276
Ptychodera, 494, 495

Ramus brauchialis profundus of facial nerve, 311

,, communicans, 2, 3
Raphe, 46

Recapitulation, law of, 434, 456, 498
Regeneration of nerves, 469
Reptiles, dominance of, 21
Retina, compound, 71

,, development of, 101

„ inversion of, in Vertebrates, 114

,, inverted, 70

„ layers of compound, 73

„ ,, in Crustacean eye, 100

,, of lateral eye of Ammoccetes, 93, 95, 111
Musca, 89

,, Pecten and Spondylus, 114

,, upright compound, 72
,, simple, 69
Rhabdites, 69, 81

Saccus vasculosus, 244, 322
Scales, 345

534 THE ORIGIN OF VERTEBRATES

Scaphaspis, 349
Schwann, sheath of, 469
Sclerotomes, 388
Scorpion, hrain, 54

,, hranchial lamellae, 175
,, development, 482

,, entochondrites, 377

„ excretory organs, 397

eyes, 75

lung-books, 150, 170
,, lymphatic glands, 423

,, muscles, oblicme, 278
n ,, recti, 271

., ,, respiration, 171

,, veno-pericardial, 177
,, muscular system, 247, 268, 269
„ nerves to Chelicerse, 237
,, olfactory organs, 220

„ operculum of male, 189, 206, 212

pecten, 359, 366, 371, 373, 374, 377
,, under surface, 372

uterus, 189, 202, 203, 204, 205, 212
Sea-scorpions, 25, 26, 27, 56, 150, 170, 191, 208, 221, 232, 235, 241, 349, 359
Segmentation, branchiomeric, 124

body-muscles in vertebrate, 388
eye-muscles, 248

of head, double, 155, 157, 173, 234, 258, 411, 459
of head-shield, 339
history of cranial, 258
Segments, branchial of Ammoccetes, 161, 178, 186

,, hyoid, in Ammoccetes, double, 186, 201, 267, 300

,, innervation of branchial, 151

,, first mesosomatic, in Limulus and its allies, 188

,, mesosomatic, of Eurypterus, 192

„ prosomatic of Limulus and its allies, 233, 249

,, ,, Ammoccetes, 286

,, of spinal region of Vertebrates, 388

,, of trigeminal nerve-group, 257, 279

„ tubular muscles of hyoid, 299

Sense-organs of Amphioxus, 34

,, branchial, of Limulus, 359, 360

,, lateral, of Annelids, 357, 367

,, lateral-line system, 356, 411, 470

Serum, 492

Significance of the optic diverticula, 102
Silurus, 488
Sinus, longitudinal venous, of Limulus, 176, 312, 451

,, rhomboidal of bird, 46
Skeleton, Ammoccetes, 126, 296, 335

branchial, 126, 126
i, ,, basi-cranial, 132

» ,, muco-cartilaginous, 291, 296, 330, 331

GENERAL INDEX 535

Skeleton, aponeurotic, 414

,, Cephalaspis, 414, 415

,, evidence of the, 119

,, Limulus, cartilaginous, 126, 136

,, ,, mesosomatic, 137

,, ,, prosomatic, 142

,, Petromyzon, 125

,, Vertebrate, commencement of bony, 120, 121
Skin, digestive power of cells of, in Ammocoetes, 58, 442
,, of Ammocoetes, 346
,, nerves of, 448
Skull of dogfish, 123

pig-embryo, 121
Slimonia, 27, 56, 170, 235, 238, 249, 276, 303
Solenocytes, 395, 477
Solpugidee, 109

Sphseroma serratum, brain, 62, 90, 101, 225
Spiders, eyes, 75

,, stomach, 109
Spina bifida, 50
Spinal cord, difference between brain and, 45

„ ,, region of, 385

,, „ termination in bird-embryo, 51
Spondylus, retina of, 114
Squilla, eyes, 100

„ glands, 422
Stomach, cephalic, 4, 43, 102, 244
Stylonurus Lagani, 27, 235, 239, 249
Substantia gelatinosa Rolandi, 44
Suprarenal body, 423
Surfaces, dorsal and ventral, 11

reversal of, 15, 29, 36, 87, 175, 352, 433, 484
Synapse, 72
Syncytium, 464, 471, 479

Tail of lizards, 50

Tapetum, 69

Teleosteans, 23, 345, 420, 424

Tendon-organs, 470

Tentacles of Ammocoetes, 246, 289, 303

Tergo-coxal muscles, 247

Test, biological, of relationship of animals, 492

Thalainencephalon, 48

Thelodus, 344

Thelyphonus, 231

brain, 53, 54, 56, 224

() ,, caudal, 450

,, coecal diverticula, 109

,, entosternite, 143

,, genital organs, 206

lyriform organs, 368

,, olfactory passage, 226, 306

536 THE ORIGIN OF VERTEBRATES

Thelyphonus, operculum, 189, 190, 206, 207
Theory, gastraa, 444, 461

Theories of the origin of vertebrates, 9, 411, 433, 457
Thionin reaction, 131, 139, 213, 330, 336
Throat, formation of, 179

Thyestes, 30, 31, 275, 326, 328, 329, 339, 340, 341
Thymus, 425, 430

Thyroid gland of Ammoccetes, 61, 127, 192, 194, 196, 429, 459
„ „ evidence of the, 185

n ,, function of, in Ammoccetes, 213

Tissues, connective, 471, 474, 481
„ evolution of, 19
,, notochordal, 435
,, two groups of, 463
Tongue of Ammoccetes, 246, 303
Tonsils, 427, 430
Torpedo, 262, 392, 470
Trabecule, 121, 132, 133, 145, 277, 295, 377
Transformation of the Lamprey, 18, 35, 59, 61, 125, 168, 193, 199, 200, 220, 227, 228,

287, 291, 304, 307, 309, 331, 336, 347, 349, 389, 445
Tremataspis, 32, 75, 275, 326, 351, 352
Trilobites, 24, 25, 26, 437

appendages, 351, 437

diagram of section through a trilobite-like animal, 413
dominance of, 26
excretory organs, 396
eyes, 74, 88
glabellum, 339
relations of, 249, 283
respiratory apparatus, 170
ventral surface, 437
Tube of central nervous system, 37, 38, 42, 102, 211, 433, 455, 457
,, from IVth ventricle to surface of brain in Ammoccetes, 209
,, Fallopian, 431
„ hypophysial, 229, 244, 317, 440
„ meeting of four tubes in vertebrate, 318, 440
,, notochord originally a, 436, 440
,, olfactory, of Ammoccetes, 219, 225, 317, 440
,, unsegmented, in segmented animal, 439
Tunicata, 16

,, budding of, 441

degeneration, 12, 17, 19, 60
endostyle, 198, 212
,, hypophysis, 425
,, notochord, 438
,, position of, 494

Unit, appendage, in non-branchial segments, 185

„ branchial, 161, 165, 168, 185
Ureters, nerves of, 448
Uterus of Scorpion group, 189, 202, 203, 204, 205, 214

„ vertebrate, nerves of, 448

GENERAL INDEX 537

Valve, ileo-colic, 449

,, of Vieussens, 48
Variation in dominant races, 21, 88

„ meristic, in spinal nerves, 154, 387
Veins, forming vertebrate heart, 180
Velum, 228, 289, 298, 302

Vertebrates, alimentary canal, innervation of, 44G
,, atrial cavity, 410

auditory apparatus and lateral-line system, 356
body-cavity, 401, 430
,, brains, 40

,, branchial organs, 151

coelomic cavities in head region, 251, 266
,, cranium, evolution of, 342

egg of, 483
„ evolution of, 11

excretory organs, 389, 391, 408
glands, ductless, 418

,, internal secretion of, 215
heart, 175, 179, 180

muscles, evidence of segmentation of eye, 248
oblique, 278
origin of somatic trunk, 406
,, nervous system, central, 13

nerves, segmental, 152
,, notochord and gut, 434

,, organs of, 10

origin of, 9, 411, 433, 457
,, segments, prosomatic, 257

skeleton, commencement of bony, 120, 458
spinal cord and medulla oblongata, 44
spinal region, 385

thyroid, connection between generative organs and, 215
„ tubes, meeting of four, 318, 440

Vesicles, cerebral, formation of, 48, 458
Vitellophags, 471, 483
Volvox, 479

Wolffian body, 390

Xiphosuba, 24, 26, 249

Yolk, 482

THE END

PRINTED BY

WILLIAM CLOWES AND SONS, LIMITED,

LONDON AND BECCLES

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